I held off for two years moving away from GMail for this reason. A year ago I decided to pull the plug anyway, and it turned out to be much less annoying than expected. My strategy was as follows: first enable a forward from GMail to your new mail address, then directly migrate the ~10 vital/daily accounts, then just leave the rest pointing to the GMail account. After that, change each remaining account immediately (no exceptions) the moment I either log in to it, or receive an e-mail from it that refers to the GMail address.
It took me about two months migrating away from GMail, and as a bonus I was able to identify quite a few old login I didn't really have a use for anymore, so I closed them.
For regular mail I put an auto-reply in GMail that says I don't use it anymore and the address will be closed at some point in the future. But honestly, I don't think anyone ever saw it as nobody sends regular email anymore these days. All in all the process was pretty painless, and I feel very happy about ditching the last Google service I was still using (except the rare Google query of DDG fails to return useful results)