yea, it can be long and difficult sometimes (And have slow progression). I have been working on a new update coming out in ~1 week where all the upgrades are much cheaper, and I added a lot of new content! (which also even involves a new currency!).
at the moment I'm not really thinking about adding checkpoints anywhere, but that would be a cool upgrade to add later. The hard thing with saving/checkpoints in this game is that it could remove the challenge with just being able to go to the last save.
I haven't worked with save files and stuffs yet so it would be challenging, but I could definitely try it!