I’d have to read a guide on how to send a patch or apply one from somebody else.

The guide is about 2 paragraphs and you'd also have to read a guide for how to create an account, fork, clone, push, send PR, etc. for the new normal workflow.

Commenting on a line of code and following a discussion about it isn’t very legible.

It's normal email bottom posting usually, pretty simple to follow. The srht UI does a decent job of this for you as well imho.

There’s no way to mark a discussion as resolved, now way to have a quick overview of the status of all the comments left on a patch

In email specifically, no. Of course you can mark it resolved if using custom software (ie, srht) that supports it. Not sure what you mean of quick overview, unless you mean via a webpage which again, srht provides. If straight email, you have to cycle through the emails. Which for me, just means typing "j" or "k" instead of page up/down like you would on GH, srht, whatever.

is it possible to submit a patch with multiple commits and if so how does one see the final result?

Yes, of course. No clue about seeing them all in one final patch. I suppose that's useful though I've never had an issue going through each patch individually. Maybe a feature suggestion for srht.

Is it possible to sign my commits?

I don't see why not.

I've used email WF, then "github WF", and found srht very refreshing when it launched. I still stuck with BitBucket because I didn't want to take the time to move over but once they removed Mercurial support, we went all in with srht and no regrets. Our code review process via email is so much faster for us now and prior to this move, I was the only person on the team who'd worked with the email WF before.

Of course, I totally get it's a personal preference and that a lot of younger developers have no experience with the email WF and humans are naturally resistant to change. They probably wouldn't enjoy it either.