Thanks for taking the time to play and review the game! It's great that you enjoyed it, and I'm glad that you could appreciate the pace at which the upgrades unlocked / became affordable. It was something a though about a lot, not wanting the player to have more than 2-3 rounds without some progress to show for it.
I think your point about the tooltip is very valid, and it could do with some more work to help legibility, although the tooltip should certainly be on top of the nodes, so that may be a minor bug i managed to introduce late on (probably just need to reorder the layers in the UI canvas a little) Thanks for the feedback.