Glad you are enjoying the game. There's a lot of routes and things to discover, most ppl find new scenes even when think all is done.
As for harem route, right now is only Anna and Charlotte, once the endings are reworked "harem route" will be tops 3 girls, ofc you still can be on solo routes with the others.
As for official relationships: Kate, Toni, and Himari (to an extent) are official relationships, Kate even having an ending.
Astrid will receive content in 1.8
As for things to spend money on, there will be more, but is not really a main thing that's been aimed for.