Researching technology or unlocking through buildings can both work in a game like this. Some kind of hybrid would be great I think, as they can balance each other.
Researching allows for improvement in performance of units, equipment, and buildings, without necessarily requiring changes in infrastructure. But it also allows for some weird things like being able to train fully armor clad knights in a newly build settlement.
Unlocking through buildings doesn't have this, and it's easier to pace I think. But it also allows for beeline tactics that are pretty cheesy and ruin games for me.
What I really don't like about research in FE is that the hated 'unlocking game features through research' mechanic from WoM is still there.
Why won't certain heroes join me until I have researched a certain tech, and then still require gold to join? Give them some interesting reasons for joining or refusing to. One could require that my sovereign be at least 1 level higher than him, so he knows I'm worthy to follow. One could require that I have a city with at least 1000 people in it, so he knows I'm trying to rebuild civilization. A few should require me doing a quest for them. A few might even require a certain tech being researched, not because it unlocks recruiting them but because they are interested in it. So one mage type hero might not join until I have researched a certain part of the magic tree, and another one might not join until I have researched Literacy because he always wanted to learn how to read but never got the opportunity to learn.
Why can't I train groups of more than 3 soldiers at the start of the game? And why can't I have more than 5 of these units together in one army? Decisions like this should be about costs and benefits, and necessity, not some arbitrary tech level.
Same thing with quests. I can understand the victory quest chain requiring research, but others not so much. There must be better ways of balancing when certain quests can be done. One quest could require a large army to beat; you can do this early but run the risk of having virtually no defences left, or wait untill later when the potential loss could be acceptable. Or it requires strong heroes; you could wait untill they are high level enough, or you could risk lower level heroes but accept that a few of them could die. Some quests could be impossible to beat untill you can deal with receiving massive amounts of fire damage. Some could require you to be able to dispell some devastating enemy debuffs. Or cast some debuffs of your own.