The playlists mentioned are perhaps hitting some kind of limitation. In Sonos usually, if a playlist cannot be loaded to a room queue within 15 seconds, it will time out.

I’m not too sure if there are any separate Plex limitations with their service in the Sonos App, but maybe make a backup of the large playlist and then reduce the amount of tracks in the original version and see if it will then eventually load to a Sonos device queue at/near a particular number of tracks to give you some indication of where to perhaps limit the size of your Plex playlists for Sonos, going forward.

Yea, I'm gathering that's it. Kinda lame it can't handle all playlists

It’s rare I use more than 500 tracks in any playlist anyway personally speaking  - plus it’s quick/easy to add plenty more playlists to the existing room queue. I can load the files locally too from the NAS and in that circumstance, I have no issues loading 25,697 tracks which is every track I possess at the moment, but loads fairly quickly across the LAN and well within the 15 second time-out period.

I want it to random play from all my music

Sonos do not recommend lists from music services contain more than 500 items.

Good for them.  I don’t recommend they don’t support them if they want people to buy their products