Simple but addictive!
About QOL changes, I found your game feel very similar to the Reactor Knockoff in some balance/game-cycle aspects to me. And I miss some hotkeys from it.
Like Ctrl+Click=upgradeAllTypeUnderCursorToSelected, rightClick=delete, Ctrl+rightClick=deleteAllThisType.
Also eyedropper and upgrade degrade hotkeys be cool.
I thought it's no so difficult to implement, but dramatically raise QOL... (it's event don't need a new UI except maybe shortcut info)