If it’s a gig then you’d probably find it easier to keep it all in one project, that’s what I do. If it’s a studio project then you should probably split it up to get better control over each song (as their sound can diverge more than a live gig). You’d still end up for example wanting the same drum sounds across all the songs but you can use preset saves for that (that’s what I do too). There’s no hard and fast rules of course. Could do some initial editing on all songs before splitting up for example.
Whenever I need to split a project, I simply duplicate the directory and change the new dir name and then go ahead and edit both projects separately. There are other ways to do it, I’m sure people will chime in. This duplicates all the audio as well, which is generally what I want - I’ll go and tidy up or delete unused audio later in the safe knowledge that I’m not effecting the other project.
However, as you’re splitting a project which uses parts of the same original audio files, I’d recommend that you store (or move) those original audio files in a separate directory outside the song project folders (but within say the album folder) with a usefully obvious name like ORIG_Audio. That way you’ll not delete it and you’ll know where it is.
Well, that’s how I do it anyway…