I played it and I must say, this game is extremely unbalanced!
There is not a single save point in the middle of the infinite waves, enemies appear in 5 from the third round. And what’s worse, they don’t give xp when they are defeated.
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 ^^'