A big pitfall of erotic games is money with nothing to spend it on.

In Theanthria right now, the main things to spend money on is fuel for your delivery pack, furniture, and gifts to give to girls. Not really a huge amount of variety.

To me, the most logical money sink to add would be upgrades for your delivery gear. IRL this is what most people who do Uber Eats for a living are already doing, apart from paying for rent, ect.

Upgrades could either be simply number increases, like 10% higher jump distance, or additional features, like a third, maybe even fourth jump, or a fuel breeder that slowly regenerates your fuel.

Another good upgrade would be something that increases how many orders you can carry, and maybe even unlocking the ability to chain orders together, like what actual App delivery drivers do?

This might clash with your idea on how the gear works, but stuff like a Hover Pack/Glider upgrade would also be pretty cool IMO.

If you wanted something really expensive goal item, a Hover Car would be a neat idea (again, much like IRL delivery.) This would basically invalidate the main gimmick of the game, so it should be extremely expensive, something you can only unlock with cheated money, or by doing enough deliveries to fully upgrade your gear.

Housing is another good money sink option IMO. Right now the default player home is a bit too "nice" IMO. Most people who enter a big city (at least in my country) could only afford a small 1 bedroom/studio apartment, let alone an entire small home. If they lived in a home, it would only be a basement suite or a single room!

There could be 2 levels of progression: Houses and Apartments

For houses it could start like:

Basement suite/Room

Small, single floor home

Large, two floor home

Manor-type home

For apartments:

Studio, Crappy area of city

1 Bedroom, nicer area of the city

2 Bedroom, Downtown

Penthouse, Highest Skyscraper

IMO I think most players would love a Penthouse, since they could directly jump off the balcony and swing down it.

The main perk for homes would be more space for furniture, as well as better starting areas for deliveries, but if you plan on having a girl/girls move in with the player, it makes sense that the player would want/desire a home upgrade (impregnation/adoption would also be an interesting reason to get a bigger home, but I don't think you have that planned IIRC.)

Speaking of the girls, "dates" could also be a good use for money. With them being broken into tiers. The cheapest being just going out to the park and eating from a food stand, while the most expensive is eating out at a 5 star restaurant. Whether a girl prefers a simple date or an expensive one would matter on their personality, and the same with the housing, with some girls not minding living in a small basement suite, while others desire the penthouse.

Again, these are all merely some brainstormed suggestions I have. Almost every H-game has some kind of issue with money, so this isn't an exclusive issue in the slightest.

If I had to make some more suggestions, it would be random activities/mini-games the player/girls could take part in, like a Karaoke/dancing mini-game at a club/bar, or an Arcade. Stuff like this helps build up the "city life" atmosphere. But these are probably much harder to implement than the other money sinks I mentioned.