Heyo!
I agree that the balance isn't quite where it should be, but there are mechanics in-place for your specific issues. This is a roguelike, so instead of save points, you should unlock Buy Ahead points, to skip ahead in the waves. I believe they unlock around waves 5, 8, 10, and 13. And instead of XP, you unlock points when you defeat enemies, which can be used in the shop to purchase upgrades.
These mechanics aren't really explained well, so I don't blame you for missing them ^^'