I don't really enjoy the current upgrade system as it's a bit linear and short - after a while it's not worth to mark symptoms for research points. I propose something in style of Crime Scene Cleaner and Baloons TD6
Each tool has its own Upgrade Tree with multiple nodes you can unlock over time.
The goal is to let you specialize your tool instead of every run feeling the same.
The top node is the tool itself (example: Mathioscope)
Nodes connected by lines are the progression path (Unlocking the left one, blocks the right one and vice versa)
Unlocking upgrades moves you deeper into the tree
Required Zone means:
You can’t unlock upgrades below that line until you’ve reached / unlocked that zone in progression.
I proposed also some filler upgrades - some nodes are empty because i didn't know what to put in there