Do you think it's a good idea to practice in custom games (kyf), so you don't risk deranking?

The term "ranked" games is a joke. If they actually wanted to make the game competitive then they should just add a casual mode where SWF is allowed and a competitive mode where SWF isn't allowed but is ranked.

For practicing? Yeah you can use it, but you learn better in public games when getting destroyed. Just don't mind people calling you bad, survivors complain even if you get a 4k. People said I'm bad for getting a 4k with 5 gens left. You don't earn any bloodpoints from KYF tho. Also you'd need a group of 5 players to get a "real" game going on.

I would say it's better for practicing specific techniques or testing certain aspects of your kit or an opposing kit.

It doesn't do much to improve your gameplay. It'll improve your gameplay against particular players, but unless you constantly "practice" with new teammates, you wont get better overall, at which point you may as well keep playing standard matches.

The only way to get better is to face BETTER opponents and lose to them. Eventually you will equal your friends. But there are people better than your friends.

Just practice in normal mode to get BP. Losing ain't a big deal anyway.
Ranks do not give anything. Deranking isn't grevious, especially since the ranks do reset very oftenly.