Should i save bps for demogorgon or level up my killers for teachables?

Don't level them equally. Get Leatherface and Hag to level 35 as soon as possible so you can have BBQ and Ruin. I'd go for BBQ first since that means more bloodpoints and therefore more levels. Then, try to get those perks on all of your killers and you'll have a much better time getting bloodpoints.

For demogorgon specifically, I'd recommend getting Save the Best for Last from Myers. Demogorgon doesn't lose stacks if he hits the obsession with Shred, so it's pretty good on him.

Surveillance from Pig would be good too. Not necessarily for Demogorgon and more for Surge, as Surveillance will work on any gens regressed by Surge. I use this to know if a survivor's on a gen without placing my portal next to one and instead putting it nearby and out of view so I can sneak up on them.

Pop Goes the Weasel from Clown is also a good perk that can go with or even replace Ruin, though you do need to remember to use it when you can.

Of course, these other perks only matter if you want to main demogorgon. So get BBQ and Ruin first, then save up BP, and get demogorgon to at least level 10. If he seems like someone you want to main, prioritize getting teachables that will be good for him. That can apply to any killer, really.