designing/developing mid to late game features

I'm developing a factory game and up to now I playtest it frequently, for now I want to focus less on finding bugs, and more in the sense of refining game mechanisms. This works perfectly and I was able to refine my mechanism, design, even changing my design (sound good on paper, but was not so good while playing...).

But... only for the early game. I need roughly 40 mins to play through my early-early game and I'm now at the point to test mid game mechanism, but here is the issue:

It is still early enough in development , that changing stuff results in corrupted or even invalid save games.

I know that some games do more technically test (unit test), use cheats, headless simuation or use excel sheets for balancing stuff etc., but this is more on the technically/feasebility/balancing side of testing.

So, what is a good/feaseable way to feel or experience your mid to late game (frequently) ?