I have the non-Pro M0 and I have 24,534 tracks in my FLAC collection. MusicBee automatically transcodes the FLACs to VBR MP3 and syncs and new ones with the M0 when I connect it to my PC (I have the same setup for my phone and tablet but using OPUS instead of MP3).
I currently have 172GB of MP3s on the 512GB microSD in the M0. Disconnecting the M0 from my PC causes it to update it's library. This happens at a rate of roughly 2,000 tracks per minute, so it currently takes 12-13 minutes for it to finish updating (I think the same task in Power Amp takes a couple of minutes on my phone and tablet)
I don't find it that hard to scroll through my library, sure it's a bit fiddly, but you can slide your finger up and down the right side of the screen to rapidly scroll through the list.
*edit* I may have over estimated the speed of the M0, it's been going for 7 minutes and it's done 11,000 tracks, so under halfway.