Is playing via “Select a music scource” preferable to saving as a Sonos playlist (or not)

It would depend more on Primephonic as to the size of their playlist. Sonos playlists max out at around 65k songs, although that is slightly variable, as it is a memory restriction, and includes some ‘tag’ information in the files as well. 

Their size is unlimited - so they say, but I cant play a large playlist on my Sonos - error 1002

How large is the playlist? Which Sonos device are you starting the playback from? Have you submitted a diagnostic to Sonos to look at to determine why it is error if out to 1002, which seems to be associated with updating, and not with playback?

While I’m generally in favor with the recommended solution on that linked FAQ, I’d go one step further, and suggest rebooting your router while the Sonos devices are unplugged, just as an extra belt and suspenders approach.