Before Nokia was assimilated and digested by Microsoft, it open sourced the OS

Huh. Did not know.

Symbian’s swansong was the remarkable Nokia 808 PureView. The 808 and other late-model Symbian devices run Nokia Belle which was really quite nice.

I still have a device here that runs that version. A Nokia 701. With AngryBirds games on it. It had an extensive app store. And that wonderful tiny charger connector (but also takes a charge from USB micro).
Nokia 701

there was that brief attempt at a revival by Planet Computers but it really didn’t stick

Oh, again I did not know that. I was interested in one of their later devices for a while, a keyboard phone running true Linux iirc.