Heck yeah, this rules! I wanted to make a Vampire Survivor like game as well, but I ended up doing something different. You nailed it though!
I really like that you can spend the gems you pick up during the run to buy upgrades. Adds just one more level of "what should I use this level up for". I will say I feel like leveling up was a bit easy. Especially once I had a few fully upgraded weapons. Might just need to tweak the xp needed to level up at higher levels. But I also liked that I level up so far, because it meant I got to see most of the game!
Great job overall! Absolutely crushed it, well done!