A bool is short for Boolean, AKA Yes/No, On/Off. Either you have it or you don't. They are not defined in any file we can access sadly, they are hard coded into the game engine.

I'm afraid that's not really possible, at least I don't think. You see the stuff in the GameInfo folder isn't actual programming code; they are files that tell the game what code to execute in certain situations (when a Starbase acquires a new upgrade etc.). You can't actually add new functions that didn't exist before. Doing new stuff typically requires a bit of imagination with the extensive ability system, but even the best modder cannot add totally new capabilities to the game engine. Since I believe no Starbase upgrades give immunity to culture overthrows, it's not something you can add in a mod. You can however have starbase upgrades that produce culture; you could combine these upgrades and let the second level of auxiliary government produce culture, thereby making the planet less likely to be lost to culture.

I'm not sure if even an ability make a planet immune to culture lose. Unfortunately a lot of the culture entity modifiers are not very well defined, so I suppose maybe extreme values could do something like that. I wouldn't get your hopes up though, and it sounds like you haven't gotten to abilities yet.

Right well thanks you two. I'll just go with the poor mans approach and use EnemyCultureRepelRate. Might even be more useful considering it's pretty hard to actually lose a planet to culture unless you're against advent loyalist

I think like most people, you're not fully understanding how culture works. In this case, culture DOES NOT change depending on the type of planet the culture center is at. The only exception is the social specialization upgrade, in which case the planets with the most specialization upgrades will be the best, but if you do not get this specialization Terrans/Desert/Asteroids etc. all behave the same with culture mechanics.

What DOES matter (and is not explained anywhere) is how many phase lanes a planet has. Basically planets produce a fixed number of culture, and this culture is divided by the number of planets adjacent to a planet, and is spread equally to all of these phase lanes. For some reason the planet infocard shows the spread to each planet, not the total culture produced. So you will get the highest culture numbers on a planet with 1 phase lane, regardless of planet type, but this culture is only spreading to one planet. An identical planet with three lanes will display 1/3 that number, but its applying it towards 3 planets, so the total culture production is the same.

As for why culture stacking often isn't OP... each time culture has to pass through a planet, it loses a certain percentage of its strength. You may have the perfect culture world in the middle of your Empire, but if it has to pass through 6 planets to get to the nearest enemy world, most of it is probably going to be wasted LONG before it gets there. That's why in general a single culture center every few planets is the most efficient for spreading culture in your own Empire. Granted you might get a planet with a great culture enhancing bonus and an extreme social specialization bonus, so that might change things, but penalty for going through planets will sooner or later catch up with it.

Culture drops 40% per jump, so after three jumps the affect, no matter how large, will be gone...that value can be modded....

If you want to make it so that each planet effectively has the same culture strength per phase lane, it is possible but you'd have to use abilities to do it...