Would it be worth the hassle, i.e any privacy/security benefit? I currently only have new sign ups on my protonmail and have kept gmail as is. I was thinking of perhaps shifting completely to a custom domain but I don’t know when I’ll do that, if ever so idk. Should I just bite the bullet and move services like my apple id, social sites (fb insta), playstation/steam, bank email etc?
Not really. It is better to make new accounts. There will still be cross-contamination even if you change the account information 100 times. In fact you’d be further doxing yourself by doing that.
I would plan and make time to redo your whole digital life with all the right tools and opsec and not bring anything over from Gmail. You can still export all your data from your Google account if you’d like but I nuked mine and did not need anything back. But your mileage may vary.
And yes, I would go ahead and get a password manager and open all my accounts and change their credentials. Or delete the accounts that I no longer use or really need.
All this work may be obverwhelming but it’s a one time thing and you’ll be better off going forward provided you maintain good opsec.
Yeah you can do that. Why not?
I mean, is there any reason not to?
Depends on the threat model and what you want to achieve.
For example if your Google account is tied to your identity then, Google can already associate those accounts to you as well and, all that changing your email would do is associate that new email with your identity for Google as @ihateKYC points out.
OTOH if you want to start organizing your digital life by using alias emails via SimpleLogin there is a benefit to that regardless.
I caution against doing this just to do this. This is where considering your threat model is important. Depending on how many accounts this could be a lot of work for marginal gain.