I'll add to this:
Nice little brothel manager, nice art. Just from their portraits, I'd like to know their traits and stories, eg shopkeeperchan.
Meaningfull upgrades other than just another room, and seeing steap increasing monthly costs additionally to the investment cost adds some deeper level of investment strategy. I find that more enjoyable than the typical debtpayoff with willinilly increasing rates.
What I'd like improvement on:
Adding girls to customers to rooms is repetitive and much clicking. As OP said, maybe automatic return after you add one couple to one room helps, or having smaller room icons (without clean information maybe) in the matching window. So you should go to room window to clean them, but you could ignore that for fast matchmaking.
An upgrade that cleans rooms between customers (might be impolemented but not found)
Some character depth, various reactions to the potions (probaply planned, when you state their flavorprofile), reactions from the shopkeeper, etc.
I don't know if your current model of "earn xp to unlock customer service cg" can compete with other brothelsims, where you can add infinite pictures and maybe unlock picture sets (positions) or player interactions. On the other hand, art is time consuming. Fleeting imagery can be found anywhere on the internet, a working game framing that is IMHO far more important.