I often think about leaving gmail, but it's not clear what the better option out there is, that doesn't create a bunch of pain in terms of not having good replacements for the rest of the ecosystem.

If it's just email, then Fastmail wins hands-down, IMO. I've been a customer for 20+ years. On my primary Google account I don't even have a Gmail account at all, but a warning if you set it up like that -- some Google products do not work at all, e.g. you cannot connect Docs to Gemini without an @gmail.com address. It will give you a prompt that looks like it came from 1998 and ask you to sign up.

I keep a separate Google account with an @gmail because some web sites don't even let you sign up with non-major-provider domains these days.

Your google account still works for drive if you switch from gmail to fastmail or proton or whatever. If you associate it with a domain you control you can even move the same email address between providers.