It takes a certain amount of effort to develop new farming methods or new ways to construct buildings. The technology points just keep track of how much effort has been expended.

I think the reason you object to the current system is that it certainly appears as though they are just doing generic research on nothing in particular, and this generic effort can then potentially be spent on any specific technology, but once it is spent it is used up and cannot be applied to anything else. This system does not seem realistic.

So instead, just pretend that you told the scientists before they did any research that you want them to work on advances in farming. When you have enough tech points to advance your farming technology and you buy it, just pretend you now tell your scientists to work on engineering.

On the other hand, one might argue successfully that people who are expert medical researchers are generally not qualified to be expert architects or farmers and vice-versa.

It is at this point someone should stop me and say "It is only a game!!"