That's a little much to say. Because setting up a good secure sever isn't trivial.

Actually it can be. There are free archiving solutions for personal use. I run Mailarchiva [0] at home and it's fantastic for archiving mail and completely free for personal use. Encrypted on disk and you don't need to run it all the time if you don't want. Just fire up the VM for a few hours a month to archive the delta. Has been flawless for me so far and the archive searching is very fast.

[0] https://www.mailarchiva.com/

How does it deal with spam filtering? This is something I understood to be a problem, along with security.

This is archiving, not a replacement for a mail server. This way you can delete all mail from your provider >30 days, for example, and still keep searchable copies off net. It's a nice middle ground and, yes, Google would still have seen all of your mail - however it wouldn't have access to it long term.

As someone who ran a significant email service for several years and dealt with many variations of discovery requests, no it isn’t.

Nobody said that you need to run a mail server, just keep your data in your possession.

If you are worried about privacy with respect to your data, you only have limited protection for 180 days if it is on the providers server. (See Stored Communications Act)

In the US, the 4th amendment ensures that you are “secure in their persons, houses, papers, and effects”. But when you put your papers or effects in the hands of a third party who isn’t your attorney, you lose much of those rights.

GMail IMO is the most honest of these services, as they are very clear that your privacy is limited in scope. Fastmail, Protonmail, etc imply things that are not as meaningful as one may think.

See: https://en.m.wikipedia.org/wiki/Third-party_doctrine