Series 3 would be Blue&Me, not UConnect.

I hit problems with the last generation (2015 180 Comp) Blue&Me system when I got to around 16GB on a 32GB stick of 256kbps m4a tracks. It wouldn't complain but there would be noticeable audio glitches/cracks/pops which sometimes sorted themselves out after a few weeks.

Rescan would take a while after adding new songs. The more you add, the longer it takes.

Are they all in one folder? I had everything in folders by artist and album, eg Drive:/Artist/Album/Track Number - Track Name.m4a.  All tracks had metadata. Sometimes it wouldn't read a particular tracks data so I would swap memory stick with a small one with a single album on, let it scan and then swap them back forcing a rescan. One issue with metadata is some long track names it would scroll the display to show it all. Others it would cut off the name. Might be metadata versions/format.

Didn't use playlists so can't say on that front.