Does that go for Bluemail, that you mentioned before? I tried it - not the new version - on one account only, so didn't see multi-account behaviour. It's pretty, but I came across a comment that it saves your mail in its cloud, which I would much prefer to avoid.
Yes, defaults to last email account used but it's a unified one by default, to choose another account you have to go through a menu, (which is at the top left of the display, naturally 🙄).
, they don't save emails on their servers.
If you activate the Instant Push facility, emails will transit their servers long enough to grab header info, (1-2 seconds for the Push notification), then they're deleted.
For some of the other optional facilities they'll store enough info to enable them to work but none of them include storing your emails.
Fwiw, I only use BlueMail on my tablet for three 'junk' Gmail accounts, (ones I'd rather not clutter K-9 Mail with).
You can get previous versions, (don't know how many), on F-Droid