Disroot and Fastmail do not use zero-knowledge encryption and of the 4 you mentioned, Proton is the only one which supports cross-provider E2EE. Disroot likely has a similar level of protection to Fastmail, but not Tuta or Proton. Whether that matters to you depends on what you want to protect and who you want to protect it from.
Using separate aliases for everything is the most private and easiest to get started with, especially if you use a paid plan. On free plans you’re more likely to either run out of aliases, experience limited features, or may be more prone to having your aliases rejected by services, but it’s doable if you’re willing to spend time and effort managing it well. It’s not a bad idea to setup a secondary (non-alias) email address for situations where a service blocks all aliases. Just make sure you log into it often enough for the provider to not automatically delete the account.
(Btw if you’re willing to pay, Proton offers both premium alias domains and multiple non-aliased email addresses, so it’s all neatly in one package. Not sure if Tuta has something similar.)
There’s pros and cons to both. One benefit to diversification could be that you’re less prone to catastrophic events such as total data loss/breaches. However, it’s much less convenient and in some cases (if you use an inferior alternative) may result in worse security. I think most people are fine just sticking to one or a few services. Just be sure to use proper security and back-up practices and you’ll probably be fine.