We kinda made the same game: Reverse Tower Defense with Goblin attackers. There were multiple others, so I made a collection.
I like how the different speeds of the units matter. If it was a little bit easier to swap units and spawn them in a selected lane, I think there would be a lot of potential for timing strategies, so you could for example send out an Ogre that you protect with precisely timed goblins etc.
Cool idea and good job!