I’ve been using Mailspring for both personal and business email, it seems like a decent UI so far, and it functions as you’d expect: runs at login, sits in the tray, notifies when new email comes in, etc. It’s open source and free, unless you need their “pro” features.
Possibly some people will be annoyed that it’s an Electron app, but it launches and runs more responsively than Thunderbird ever has on my machines, so I don’t find that to be a problem. I would rather a Gnome native app, but I’m not aware of any that function well, as OP laments.