Here is a possible solution. It requires the mod Custom Barn Kit, which allows you to edit the number of tiers in KSC buildings, and the level at which upgrades are available and other related factors.
Change # of tiers of tracking station to 4
Change unlockedSpaceObjectDiscovery to 4
Don't upgrade any further.
I don't actually know if it will change things that have already been upgraded.
Good luck!
Nightside's post in Rolling back the Tracking Station to tier 2? was marked as the answer