Fantastic! Extremely impressive as a jam project, I'd love to see an expanded release!
Being able to upgrade existing buildings/drones would be number 1 on my list, with clicking efficiency upgrades as number 2.
Otherwise, pie in the sky/scope creep idea, have a secondary game screen that's a planet map. Instead of building an HQ to end the level, you are building a launch pad to get to the next planet. With each planet completed continuing to contribute resources for unlocking new passive upgrades.