Well… it’s quite easy to do what you’re trying to do in the regular library view (not in folder mode though).
There’s an existing feature suggestion asking for an option to select all items in a folder (in consequence allowing all of those to be added to a playlist in 1 go)
In addition there’s already an existing suggestion asking for an option to add entire folders to a playlist or the play queue.
I suggest you comment/vote in those existing threads to avoid distracting votes.
Therefore I’ll close this thread as a duplicate.
