It's not an easy task! I had mine account since gmail beta, so much stuff is going through it.
Two years ago I moved to Fastmail(they're great), and pretty much setting up an alias per site/purpose. While I'm happy to see that gmail receives less & less, I still have about 150 entries in 1password that say @gmail.com... maybe one day...
Just do email forwarding and progressively update your accounts?
I know! I got my own domain and it probably took me over a year to be confident that no important email was getting to me though my gmail handle I had been using since 2004
It's a very easy task. Move your stuff and accounts to a different provider in the pace you prefer, and set gmail to forward to the new provider. Keep your gmail account around as a haunted house for the occasional messages that can only come through that way.
Compare it to somebody who eats fast food every day. They can start eating healthy instead, but it doesn't mean that they are never allowed to eat fast food again, or even once per week if so be.
I switched to migadu which was not too painful for a single account (basically the advice was to put both accounts in thunderbird and then move everything over, thunderbird does the work). For email very happy with the provider, notably also as it allows infinite domains.
But I miss a proper calendar which migadu is strangely mostly lacking (basic caldav only). Is fastmail better in this respect? And can you do wildcard emails and send/receive as whichever name you want with fastmail? E.g. I have it set so every email which comes to xyz@domain.com arrives in my single inbox (so not catchall but all with a certain letter combo) and use this to have custom emails for every site.