I should expect that uninstalling my mod would require the player to first remove all existing TechMining infrastructure?
It's actually worse than that - to allow uninstalling, all existing tech-mining would need to be removed, including any tech-mining on NPC planets, and then the game would need to be run for two months in-game time, otherwise the end-of-month cost summaries will crash the game looking for your implementation of tech-mining.
If you want an uninstall-like option, the best I was able to do was to include a settings switch that restores vanilla behavior.
What if I set TechMining to be a Structure rather than an Infrastructure?
What might that do to existing Infrastructure, or (on removal) do to a Structure suddenly switching back to Infrastructure when the planet is already maxed out with Infrastructure?
Setting tech mining to be a structure rather than an industry will just work.
Setting it back will also 'just work', but planets that find themselves over the industry cap will suffer a -5 stability penalty. (This stability penalty is a vanilla mechanic, intended for the case of "Someone gets a hypershunt tap up and running, builds an extra industry, and then has a shipping shortage on transplutonics".)