Strawberry or Clementine. I mean 100K entries in a database is nothing. Even for SQLite. You can add multiple library locations, this is no problem.
You probably want Strawberry as it is newer and maintained, but I still like Clementine for the extra features that Strawberry doesn’t have yet. For you probably, not a big deal - things like podcast support, cloud support etc.
Thunderbird. It’s great
I am not sure how to make it look shitty like Gmail, maybe you could theme it to wast a ton of space.
Seriously, do you want a useful email client or not?