I’m actually having success with the Mail app on my S4 Apple Watch. Looking back at how horrible Mail was during iOS 13 (Waking
every morning to see a 2 badge number and then opening Mail and it updates and I’ve got 6 or 12 emails. Always a crapshoot), I never even really tried using the Mail app on my Watch. I’ve found it to be quite useful lately.
Initially, to get Mail working for me again, I had to remove each account including my iCloud mail, then delete the Mail app, then reinstall and add the accounts back again. But from
everything I’ve read about the Mail app not working, it’s really going to be once of those YMMV sort of things.