Email issues on iPhone 12 Pro Max

Having some email issues with my wife's 12 Max:

1. It's an IMAP account. I have email coming from the same server/domain on my phone as well (an 11 Pro), using the exact same settings, and I've never had a glitch, so I'm pretty sure it's on the phone end, not the server side.
2. Sometimes, the email icon will show an insane amount of unread messages, but when clicking to the folder, there's nothing there. Or maybe there is, but they're all read.
3. At some point, it will just stop receiving new email, but still show (or not), unread messages.

Usually, I can fix it for a bit by turning the email account off, rebooting the phone, turning the account back on, reboot the phone again, and then it's straight. But, only for a few days, then it's right back to where it was.

I've also completely deleted the email app, reinstalled it from scratch, and entered the account again from scratch. Worked for a few days, then back to being weird.

I've just turned PUSH off, and will try fetch instead for the first time, but I don't have high hopes.

Ideas/suggestions?