I will update X2 to something better. Balancing is still tricky, but let's crank it up and see. Ideally it should keep the early upgrades relevant, without overpowering the later ones.
I would like more Rebirth upgrades but I don't have any good ideas atm. They should also be not-too-hard to implement, since I will have less time soon. Also, balancing gets really hard here, since it is so far that a naturel progression test if not feasible.
It feels almost complete, but only almost. I'd like to add more in the future, but it would be at a slower pace. Glad you enjoyed it! Wouldn't be as much fun to make if nobody cared.