I know tech trading from galciv2 and I hated that. It was just so overpowered mechanic you HAD to constantly follow and micromanage not to fall behind, constantly checking all diplo contacts every ten turns or so and buying all techs you can. It trivializes the research game completely, you either have that or some more refined and interesting research system. Also, it wasn't that ruinous for gal civ because the game had a ton of technologies which mostly gave not so huge individual advantages (lasers 5, lasers 6...) - in civ an ability of you and your enemy to suddenly instantly buy out devastating military technologies would really disturb balance.
This is matter of move the numbers, find the more balanced conditions. And some additional elements related to
POPs:
-
OCCUPATION each pop have a job that affect how useful could be the new tech, for example a nation of mainly farmers would find hard to assimilate new naval techs, while a nation with many scholars would certainly have curiosity on new optic instruments.
- The
IDEALS of your pops and the
POLITICS of your government would also play a role, basicaly some civs would be too traditionalist to just absord new tech as the same speed.
It doesn't even make much sense IRL. Non - Europeans countries were 'quick buying' guns and advanced products, but they weren't able to just 'buy' the ability to have advanced industry necessary to create them yourself, which is sort of what tech tree in civ games simulate I guess? Not to mention 'buying' the Theory of Relativity or theoretical stuff like that. It's not like science like that is (usually) super secret you need to buy, it's that you need awesome education & research to be able to fully understand its and explore its consequences. Which is again kind of what tech tree simulates, though you could complicate the process more.
You said it "advanced industry" Haudenosaunee population and infrastructure is not the same as Sengoku Japan. Now on game civs like Cree are not really supposed to be like their real historical version, by time of gunpowder they would be also centralized, imperial and urban society so the whole idea of "primitives with guns" is point less. Every civ on CIV are just "flavor theme" over euro-history design.
Anyway you want the game to be more realistic? Allow to have
Guns as a strategic resource that could be produced on worshops when you have the tech, then link the training of "gunners" to either have and ally that produce the resource or make it by yourself on yours workshops.
About Theory of Relativity also the general idea is not a secret, on the contrary recent techs are the ones that propagate the faster. Now what you can do with those knowledge is different, but again depent of others factors. In this case are Nuclear Reactors and Nuclear Weapons. On game you need:
1- Theory of Relativity (the Tech)
2- Uranium (a rare Strategic Resource)
3- Reactor (very expensive and could be dangerous)
4- Nuclear Device (each one should cost
Production and
Science representing the great amount of industrail capacity and research specialization to produce it)
5- Missil Silo (also the expensive result of others techs like Rockety)
So the real significa value of the tech is not just on the idea but on the capacity to build and use their products. A tech focused civ would still have the advantage to produce high tech units and boost their industry. While the pre-industrial / pre-scientific techs would also naturaly regulated by their kind of society.