I wonder if 'flipping around' the requirements might work?
So instead of the chosen planet type dictating the temperature, the existing temperature would dictate what types you could change to.
And the mirrors/shades can modify the temperature by +/-1 and so alter the 'allowed' conversions to include more (favourable) types.
This would narrow the scope of potential terraforming options for any given planet, but allow the player to expand that by investing in infrastructure. But only upto the limits of what changing the temperature 1 step would allow.
For example:
If I had a super-hot volcanic world, the only terraforming options I'd have right at the start would be: Volcanic, Toxic, Barren.
The 3 (major) planet classes which are allowed to be super-hot by the procgen.
Building shades would reduce the temperature by 1 step to 'just' hot, which then opens up additional options for Desert, Arid, Jungle, Water, Eccentric.
Aka, all the types that are 'allowed' to be hot.
And that's as far as that particular world can be terraformed.
This would also give you the option to de-restrict terraforming options in the settings to allow mirrors/shades to unlock any transformation or make them entirely optional.
The point based system could stay in place along with all the current means of generating said points.
There would however need to be a check in place to look out for existing infrastructure as there are now instances of worlds with pre-built mirrors/shades which probably shouldn't allow the player to build any of thier own.
The new implementation in vanilla is to "suppress" negative conditions like temperate, poor light, etc., not to remove or change them. This is how the fusion lamp and the vanilla stellar mirror condition work (although apparently Eventide and Eochu Bres don't have any of the conditions that get suppressed, so this isn't immediately obvious without testing the fusion lamp).
I want my implementation of terraforming to conform with how the fusion lamp works in vanilla so players can understand it and it "gels" properly. Instead of removing or altering negative conditions, I want my terraforming buildings to use the new "suppression" feature instead for most situations (exceptions being pollution and decivilized subpopulation).
For example, instead of removing the Meteor Impacts condition, I'll have the planetary shield suppress it. For No Atmosphere, I may have a dome-city building that will suppress it, but not remove it, so that the hazard increase is gone but the special items that require no atmosphere will still work.
One of the problems I found with my current implementation is that it results in very boring and identical markets - Terran worlds are strictly better than all other types, so players just create a bunch of identical 50% hazard Terran worlds and nothing else, and there's no tradeoffs or choices to make. Whatever I decide to do, I want to try to avoid creating this issue with the new system. The special items that require No Atmosphere to work means there are now two "ideal" planet configurations, not just one, which isn't much of an improvement.
The idea of limiting terraforming to one "temperature step" is interesting. It seems to work well for hot worlds, but the fusion lamp causes problems because it suppresses both cold and extreme cold, even in systems with no star, and it will make planets hot if you don't meet the volatiles demand. The "temperature step" system would seem to only place restrictions on Very Hot planets, while all other planets could easily be turned into whatever the player desires, provided they have a fusion lamp.