Gmail on Android complains when a mail server cert changed (good) but doesn't let you proceed to say it's ok (bad).
Let's Encrypt's certs are valid for 3 months.
EVERY THREE MONTHS I HAVE TO REMOVE AND RE-ADD MY MAIL ACCOUNTS
...thanks, crappy Gmail app -_-
Don't worry! It's not your fault at all that the app is not coded to handle something so simple. You totally did the right thing by changing the certificate to one with a chain with a trusted CA. If it was a problem with what you did, I'd have come to you. But the problem lies with the email client.
Thanks for the recommendation, you're the second to have mentioned that one. I'm trying out Outlook first as apparently their Android client is actually really good. Will report back!