first thing first i really liked the game its quite fun, my favorite map was the "ice" one and i beat the final level with a combo of flame throwers and cannons. i liked managing the barricades.
but there's a problem and it is the energy and the gun caps which do the same thing, if you want to differentiate with "we have an orc problem" i would stick with energy since it has a lot of potential for flexibility in which guns you can field and other game mechanics like something mentioned in Fort Kenmei's video something like putting generator in the tracks and idk making it power some turrets locally and give them like 10% more damage. that way you could have like layers of defense.
Also instead of making gun caps i would have energy upgrades and energy efficiency upgrades that reduces the cost of the guns so you can field more.
If you care about it being too easy i would say don't worry the thing about this incremental games is that you can go at your own pace. if you want to introduce some challenge something like achivements for not dying would be good i think.
also i didn't like the sound the spaceship make when collecting money it got repetitive