Very simple idea to fix tunneling for both sides

I don't think this alone would be effective enough honestly, especially considering the downside doesn't apply as much if you don't run 4 regression perks and run a more aggressive build. And even then, people will run perks like TBTC which even at 5% will be almost just as effective since you're still remotely breaking a gen, losing no time and no distance.

To solve tunneling, it primarily needs to be completely disincentivized leading to spreading hooks, as opposed to incentivizing spreading hooks and hoping killers don't tunnel. I don't know if there's really a super simple solution to do this without harming killers who simply need to get a survivor out at 2 gens for a chance to win, but either way BHVR has had 10 years to figure something out and still hasn't done anything.

As for Pain Res, just make it disable when a survivor is killed.

Well, since there is no answer I will elaborate myself.

There are currently a total of 0 benefits for killer for spreading hooks. The pressure you gain is same as if you did with other strategies, perks provide same value, and you provide much more value for survivors if they have hook related perk, some of which are meta, like resurgence or decisive strike.

Discouraging tunneling by pretty much disabling it will lead to Hook spread? Do you really believe this? It will lead to meta shifting from tunneling to hard slug one. Meta for killers will always be the most efficient strategy, and hook spreading is the last in the list, it has always been the last in the history of ever in DBD. And you DO NOT want a hard slug meta, especially with now disabled abandon for excessive slugging.

The benefit of spreading hooks is that it slows down the game. It's an efficient use of your time and resources.

It only seems otherwise because tunneling is so unbalanced you can play that inefficiently and have a higher chance of winning.

To the wider topic, actual basekit adjustments need to be made in order to move away from tunneling. Perks aren't needed for it to be as strong as it is, and if a super strong, super easy and free strategy is available, you can't fairly expect players to opt out of using it if they want to win.
Nothing wrong with these changes in addition to proper anti tunnel, though. Hell, bring back the stacking BP bonus for fresh hooks as well, I liked that change a lot- just none of these things can fix the problem on their own.

If we were to imagine a state of the game where the chases you take to tunnel a survivor out were even as long as any chase you take outside of tunneling, then spreading hooks would be the vastly superior way of slowing down the game because you'd actually be interrupting the whole team's objective progress instead of leaving half the team minimum to do gens unimpeded.

Targeting just one survivor should be inefficient, since you're electing not to pressure the team and instead focus only on one person. The only reason it isn't, or at least isn't seen that way, is because the chases you get are much easier and therefore noticeably faster.

Taking a chase on a survivor that was just unhooked gives you a lot of value while asking nothing in return and leaving very little room for the survivor to respond without specific perks. It's because of that, and therefore tunneling being very obviously just a balance problem, that we can correctly understand 'spreading hooks' doesn't need to be stronger— 'spreading hooks' is the entire video game outside of specifically tunneling and camping.
Tunneling just needs to be weaker, or more ideally removed outright. 'Spreading hooks' is only seen as objectively the weakest strategy because tunneling is so unbalanced it warps people's perception of the game. Hell, it's only seen as a single strategy at all because of tunneling.

I apologise, I know you wanted to look past the discussion of tunneling as an issue but it's very integral to a proper understanding of what to do in this context.

It is not "seen" as weakest, it is objectively weakest. You are correct, chasing a fresh healthy survivor in an area where chase didn't occur yet and resources are still present, as against chasing a wounded survivor in an area that is likely to have some resources already spent.

However, that's not the only issue with hook spreading. Other strategies have the benefits which makes their playstyles work better to apply pressure.

Tunneling benefit: the game reaches 1v3 the fastest. 1v3 favors killer much more than 1v4.

Camping benefit: You might create pressure via forcing a hook trade, in which you greatly reduce the phase of reapplying 1v4 window scenario. (I don't know how the 1 chased 1 on gens 1 saving and 1 hooked is named) Or you create pressure by possibly forcing 2 to go for a save, which also extends said window.

Slugging benefit: You skip the hooking phase entirely. If you have map traversal killer, you also increase the time people stay wounded by constantly returning to slug pick up.

Hexes benefit: Alternative objective which might even remove the generator survivor from the 1v4 window.

None of these benefits are present in hook spread. It is objectively the weakest strategy because it does not provide any benefit of it's own, and doesn't beat others in pressure applied.

In truth, the best strategy of a killer is still the hard tunneling, because 1v4 window is not present in 1v3. 1 survivor is no longer on the gens, effectively reducing the efficiency not by 25%, but by 100%. The only way to stop that is if the 1 getting chased is not going down, or going down after a long time.

And no you are correct tunneling should be discussed in here, though our arguments will be affected by our viewpoint on it.

It is still very much a strategy, I never said that it is the only strategy to be used throughout the whole match, that would be wild to claim. I am talking about hook spread from the 1v4 Pov. I will admit, I made a mistake, there is one benefit to hook spread strategy: It is one which focuses the most on using the window situation to pressure important for later generators. After people tunnel someone out, what usually follows is exactly hook spread, while one is hooked they go to the generators to protect them and hook whoever remained to repair them.

In 1v4 however, it is the weakest because you would just be throwing the game. You do not have the leisure to just go from hook to hook by traversing big chunks of map to check up generators and chase whoever is on them. A lot of time during that window is spent on exactly map traversal, and the window situation always has 1 repairing a generator.

Hooking is essential to the game, but just doing the action doesn't mean you are hook spreading. You might as well slug all but then one guy will be hooked all the time. In that case, you are slugging and tunneling at once. Or in case of hex or hit and run, you might occasionally hook, because either your hook is close to hexes so you can camp and defend hexes, or all are wounded, so saving is now risky and camping comes in action.

A rather close game between killer and survivors most likely would demand a constant mix of all the basic strategies killer can use, hook spread, tunneling, camping, slugging. Using them on occasion but never dedicating to one. Depends on conditions tbh.

It is why I consider hardcore variation of tunneling camping and slugging to be extremely unhealthy to the game, but those that are used occasionally are perfectly fine.

You yourself literally just described what Hook spreading strategy is: leave hook to go chase someone else and not return to tunnel if they are unhooked. If you tunnel, it's now leave hook to go chase someone else but then return to hook if they are unhooked. If you camp, it is you don't leave the hook too far, and remain close. If you slug, you leave right away and maybe return maybe not. If you hex, you hook to distract 2 from people cleansing. Yes the action of hooking happens in many of them, but spread hooking itself is a separate strategy, it can occasionally happen even during tunneling if you left to chase someone else and downed -> hooked them so fast the first one wasn't unhooked yet.

The backbone of everything for killers is to keep creating the time window situation and make it as effective as possible. Hard tunneling eliminates the very need for such situation as fast as possible because game turns into a 1v3. Hook spread maximizes the time needed for thus time window to keep existing, while not doing much to make said window effective, which is why other strategies usually come into play.

Hooking, the action itself is the essential thing to many strategies. Hook spreading is exactly it's own strategy. Find survivor A, down them, hook them, find survivor B, down them, hook them, repeat. Why would I focus on chasing someone else I don't know where as I am currently losing pressure? I would much rather use the stronger options, since hook spread doesn't reward me with pressure needed anymore