Yeah I do think a more interesting upgrade system could add more replayability and a wider variety of interesting strategies. I enjoyed finding different ways to build a good DPS tower (e.g. "multishot + explosion" vs "pierce + returning + farshot + poison" or whatever). I think if there were more buffs that interacted with the other upgrades that might help, maybe along with giving you more control of which upgrades to purchase.
I also considered making tower build cost scale much more aggressively with upgrade count so that you felt really incentivized to keep one tower unupgraded to build your maze, but I didn't like that because buying upgrades is the fun part of the game.
But glad you found it fun (and thanks for leaving feedback)!