It’s really nice for the transition period. I personally forward my email to Tuta, which lets me slowly convert my services to my new address. I have my most important ones switched over now, but I had to switch dozens over (I would do 3-5 at a time, which was a pain).
I’ll probably leave my gmail forwarding to my Tuta account, just because there’s no way I’m going to go though every single service I have ever used and switch it over, and inevitably some contact will continue using my old email.
As far as Google goes, all it knows is that it’s getting less and less emails, and that what remains is being forwarded to <email>@tuta.com. But that’s not my main email address though, it’s just the one I set the account up with. I actually use <name>@<custom domain>, and I have a bunch of aliases configured for each type of account (e.g. <name>-banking@<custom domain> for my bank accounts, <name>-bills@<custom domain> for utilities and whatnot, etc). But that’s still not my actual, personal email, which is <name>@<different custom domain>, and I only give that one to my family and friends.
So in short:
gmail -> tuta.com email - all Google knows about
random online accounts -> custom domain 1
family/friends -> custom domain 2
If I can convince my SO to switch, I’ll give them an account at custom domain 2 and tell them to only use it for personal contacts, and to have everything else go through their old gmail or a Tuta alias. If I ever decide to switch to Proton, I’d have to transition all of those custom domain 1 emails to some proton aliases (unless I pay for the higher tier), which would be a pain, especially since the main reason I use these custom domains is to make it easier to switch services (e.g. just point my DNS records to the new host).