Not OP, but using a similar setup: iCloud for government, banks, and a custom domain with Proton Mail for everything else. A few major banks here in my country have hardcoded domain names to include only Gmail, Outlook, Yahoo, iCloud, and a few local providers. The same is true for the Income Tax portal. Out of the given options, iCloud is relatively more private.

I’ve encountered a few other websites doing this kind of lazy implementation. Hide My Email by Apple works in such cases too, where I don’t want to disclose my primary iCloud address, which is accepted globally especially since it ends with @icloud.com.

Personally, I use it through Safari with Wipr 2. Wipr’s developer is pretty active and responsive too. Another option is using Adguard Pro. Vinegar is a good option too, but it’s developer suggests us to use a different adblocker and use Vinegar for it’s main purpose of forcing the native HTML 5 player. If you’re okay with Brave’s crypto shenanigans, it works well with YouTube too.

I use YouTube almost always signed out. Along with icloud private relay enabled, it does minimise the amount of data that Google can collect. I would prefer to use Invidious or Piped, but they bug out quite often with YouTube playing a cat and mouse game.