The biggest issue I see is that there is no threat of losing. The enemies just stand there waiting for you. The enemy should push forward and attack, even if there is a delay, or they move slower once reaching a certain point. If defeated you could restart the same level with all the same resources, or maybe a penalty for the loss. There are plenty of options for what to do on loss, it just feels like there needs to be something. You have a health bar on the base, so it seems like there might be some plan for the enemy attacking.
Similarly, I know its an idler game, but once you beat a base you can just walk away while accumulating unlimited resources of that type. When you come back you can max everything of that resource out, then go to the next and repeat. Going back to the point above, if the enemies could attack you, and you had a penalty for losing, that would limit this somewhat. As an idler, I'd expect that on loss it would auto-restart, possibly after a bit of a delay. You'd still be able to accumulate resources, but make it so it is more reasonable. Perhaps even add some limits based on human interactions, or on the currently unlocked upgrades. Maybe make it so that you can not get more resources than needed for the most expensive upgrade using that type... well, maybe make it 2x or 3x the max so you don't lose resources while actively playing and just hadn't gotten to purchasing the upgrade yet.
I don't do many idlers, so perhaps my thoughts aren't as relevant to what idler players want. I played to the end game screen then did endless until I maxed all the upgrades. Overall, it feels like a good start.