Like Hentaignome said, if you get a skill that generates passive income and leave the game running for a while you can get infinite money very, very early-game.
That money will help you progress quickly through the game, such as by purchasing all of the camera upgrades.
If you want active progression quickly, I've found the best bet is to unlock demon-hunting and get the gold Katana. It costs $6,000, but will earn back that $6,000 every day you hunt, including the day you buy it (I can't remember if those are the exact numbers, but it is a surprisingly large amount of money.)
That all being said, the developer doesn't just add content to the end of the game whenever it updates. He also frequently adds content to the beginning/middle of the game as well. I personally replay the game from the beginning whenever a new update drops. If you want to see the new content in the next two updates, it isn't worth your time to create a "completed" save file, as you will probably need to replay the game again when it releases at the very least.
So for now, maybe just focus on the girls you want to see scenes for and wait until the next update to sink all 20+ hours in again. The playthroughs get faster after your first.