This game is big numbers, and definitely a lot of fun for the first few playthroughs! I found that the "end round" button was actually a key part of the strategy toward getting huge numbers, because it means you can leave more tiles open for the pieces you want to place as the rounds continue. This makes the game incredibly easy once you start intentionally ending the round with lots of tiles left over and have an engine built around 1 or 2 territory types. I'm currently on round 42 and am able to win the rounds by placing only one piece per round because I've buffed up my mountains enough that this is possible. I don't foresee any end to the game at this point, and my point total is currently in the trillions for each round, as well.
A couple ideas that might help the game feel more challenging:
1. Adding an "antagonist" that maybe fills unused tiles occasionally. Or some sort of negative events that slowly disable the tiles you currently have on the board?
2. Scaling the price of tile rerolls in the shop with each reroll. Maybe even in a way that extends between rounds - meaning if you rerolled in a previous round, the price of a reroll in the next would be scaled higher.
3. Having some of the "buyable" tile pairs have at least one "non-territory" square (i.e., one that doesn't do anything for your points in any situation), especially if the other tile in the pair is powerful.
4. Maybe this is a reach, but having some "special effects" on tiles have a downside - i.e., instead of "Territory Type get +2 permanent points", something more like "Territory Type gets +2 permanent points, and Other Territory Type gets -1 permanent points", or similar.