My gripe with using iCloud for my email is that I just am not a fan of the mail client. Because of this, I've been very reluctant to send emails with it and prefer using my personal Outlook.
I use my personal email with the Outlook client, but I link this to a Google account mostly for convenience. I feel like switching entirely over to Google Workspace would be nice, but (1): Lock in (2): I don't think Workspace supports the catch-all/many email addresses for 1 user (3): I don't like Gmail's client. Switching over to Microsoft 365 has problems 1 and 2, just not 3.
What is the best solution for running email, preferably FOSS?
Lock-in is not so much of a problem I'd say, you can export your entire inbox, that's possible with both Google and Outlook that I am aware of.
Just about any email service will offer IMAP, Google included. So you don't have to use the Gmail client if you don't like it, you can install any email agent of choice.
Don't try to self-host e-mail. It's not worth the headache.
I used to be on google workspace, but I moved away from them after the cancellation of the free tier fiasco a couple of years ago.
Migadu comes with a web client, and it's OK I guess, but I use Thunderbird and an IMAP connection on PC, and the google mail client on Android. Happy with it so far. I do miss Google's powerful search and organisational capabilities though.
Fastmail do have web and mobile clients. On web and iOS, both are a "webapp" disguised as an app (my guess Android version is the same).
That's part of why we're building Marco:
I'm building it for myself, but sounds like it could be a good fit for you too.