*laughs in convulsions*
One week? I doubt it is enough, "crazy" tag is there for purpose. Still, it is looks like there are a solution for 12 buildings hardcap. At least partial solution for terraforming buildings. But all in order.
1. Terraforming options impossible without terraforming platform. (TP for short. The main thing is not to be confused with Toilet Paper.) It is constructed like an ordinary building, but disappear when finished (like an astropolis). Parent planet (PP for short) now has a "teraforming platform" option in planet menu. It leads to separate imaginary TP market. It is also available from colony screen for building remote control if necessary. TP market has size 1 (or even 0, if possible) and does not have "population and infrastructure" (will not grow or consume resources for that building), it shares accessibility and ground defense value with PP. Does not generate any fleets (fleetsize=0) Ordinary buildings are not allowed at TP, only terraforming buildings and projects (except of Ismara slingshot). Terraforming buildings are not allowed at PP any more (except of Ismara slingshot). TP could be as station around the PP or better, not to have real object on system map, i.e. being imaginary (if it is possible), in that case all trade fleets and enemy expedition should be canceled for TP or (better) redirected to PP.
2. TP created with custom market conditions, like:
"Initial temperature - extreme cold";
"Initial temperature - cold";
"Initial temperature - normal";
"Initial temperature - hot";
"Initial temperature - extreme hot";
"Initial temperature - no atmosphere";
"Initial temperature - thin atmosphere";
"Initial temperature - thick atmosphere";
"Initial planet type - barren";
And etc. Those conditions does not do anything, it is just a reminder for player (and a mod) where it should deteriorate over a time if TP abandoned or some TP buildings sold.
3. Teraforming projects is a replacement for a teraforming menu (BTW in this system it is obsolete). Each project will change planet type for desired. Can be picked in construction menu. They have progress bar like population and infrastructure, but it is applied for (current terraforming points/required points) rate. Can not run more than one project at a same time.
4. Terraforming buildings generate points (as it is now) AND are mandatory for starting some projects. Example: current temperature is "normal", so it is not possible to perform "Ice conversion" project without stellar mirrors as it requires either current temperature as "Extreme cold" OR stellar mirrors to be present (big red tooltip text "build stellar mirrors first"). Even if project finished, removing mirrors will cause slow deterioration for initial temperature AND initial planet type if it is not compatible with initial temperature. If PP current temperature = TP initial temperature there would not be any deterioration, but mirrors still can be used as points generator. Mirrors will not change the current conditions without project, tho. Absolutely same thing for atmosphere and Atmosphere Processor. If initial type does not contain much water, requires Ismara slingshot or asteroid processing to be present in system (asteroid processor is available in TP just in case player can not build any station and there are no water planets in system, removing asteroid processor later will not cause deterioration). "Arid" project does not require that.
5. As it is not possible to get arcology other than terraforming, why not locate it's unique buildings on TP?
That is not all. I just tired of typing...