Hey, thanks for commenting. I really appreciate in depth feedback like this.
Honestly I didn't have a lot of time to test Endless + Hard, I figured an issue like this would pop up but I didn't think it would happen so early into the mode. These things will absolutely be getting tweaked and balanced in the next major update. I already have plans for improving the issue of running out of things to build and spend resources on in Endless mode. There's also a system planned for dynamically reducing the number of particle effects with large numbers of units on the screen, that should also be in the next update.
Thank you for supporting the game.