I guess to account for such situations, you would need to put crossgrades at the ends of the slider spectrum. So for example upgrading into Cilia would have to be on the short extreme end of the Flagellum upgrade map.
The reason I’m tentative towards your replacement idea is because it sounds to me very similar to the above system, except simply detached into its own system.
By the way, I’ve been drafting a list of the possible upgrades based on ones we’ve discussed in the past. Here’s a draft list so far:
No worries, been swamped with classes myself otherwise I would’ve answered days ago. My best advice is to make sure to take some days completely off. If you have no “weekends” or “breaks” from Thrive you’ll burn out. Plus, I find that by the end of those weekends or breaks I’m 10x more eager to return.