don’t pay for Spotify but i’ve currently only got a 2GB memory card for my phone, cos the 64GB one i bought doesn’t work with it and there’s no point getting a 32GB to tide me over cos i need to get a new phone soon anyway.
so i just swap the music over every few days or once a week at least. tend to load on anything new i’ve bought which i’ll keep on there for a while, and pick out old ones i fancy listening to that week which will usually get deleted off again after one listen, two at most.
not sure how it works with Spotify (i assume you mean downloading them to play offline?), can’t you delete and replace the albums regularly after a couple of plays?