Regarding your proposed changes, I can see a few potential issues:
1. You write that the focus for Terran is farming - but since it starts with volatiles and therefore can't slot Soil Nanites, it's actually inferior for farming relative to Jungle and Arid despite their lower max farming values!
2. If the player suppresses the Hot or Cold condition, I think they'll have the expectation that they should be able to turn that planet into a Terran world. Stopping that wouldn't make sense from a lore standpoint. Also, I think all procedurally generated Arid/Desert planets are hot - allowing the player to create Arid/Desert worlds that are cold seems to be incongruent with how the vanilla system works, even if it makes sense from a realism standpoint.
3. The divergence in max values between planet types is less than the current system (even for farming, if you assume the player has access to Soil Nanites). If anything, this will make the planet types less distinct and more interchangeable.
1. I consider soil nanites as form of rare compensation for worse conditions - not a default bonus. I played many games without even finding one. But for compensation there has to be reason for it. I was able to create a jungle food +9 paradise (food +2 from bountiful farmland, another +2 from reflectors, and +2 from soil nanites, +1 from governor, +1 from alpha core, +1 from story point improvement) from barren extremely hot planet. It was hurting my eyes! I hate soil nanites.
2. Well, I was thinking about that too, but currently Arid type has little reason to exist, because it is outclassed by Jungle and Terran (no one cares about traces of volatiles). I was thinking about making Terran type a bit elite but... I got another idea! Terraforming points!
Each planet will have a pool of f.e. 120 terraforming points to spent. Every action will have a cost. Every planet and resource will have a preferred temperature condition:
Terran - normal, Water - normal, Jungle - hot, Arid - hot, Tundra - cold, Freezing - very cold
Food - normal, Organics - hot, Volatiles - cold
Base cost for resource improvement could be 12 points (12 is nice number; can be divided by 2,3,4,6), but if the action is taken outside of its preferred conditions, to the cost will be added another 12 points for every temperature difference step. Suppressing conditions by solar mirrors/reflectors will halve the penalty.
Example 1: Improving volatiles on hot Arid planet will cost base 12 + penalty for volatiles hot to cold difference + 12*2 = 36, but with reflectors: 12 + 24/2 =24 points.
Example 2: Improving food on hot Terran planet with solar mirrors will cost base 12 + 12/2 penalty for food normal to hot difference + 12/2 penalty for improving not normal Terran = 24 points
Example 3: Improving volatiles on very hot Jungle planet will cost: base 12 + 36 penalty for volatiles very hot to cold difference + 12 penalty for improving not on the hot Jungle (no suppression can be made) = 60 points
Example 4: Improving organics on very hot Jungle planet will cost: base 12 + 12 penalty for organics very hot to hot difference + 12 penalty for improving not on the hot Jungle (no suppression can be made) = 36 points
Example 5: Improving food on very hot Jungle planet will cost: base 12 + 24 penalty for food very hot to normal difference + 12 penalty for improving not on the hot Jungle (no suppression can be made) = 48 points
Atmospheric improvements also should have a cost f.e. 12 or 24 points?
Changing planet type - 48 points?
Terraforming points should prevent creating extremely hot/cold monstrosities that i mentioned before. A hope it is doable.