I think what I might like best would be that traders when they move give you a free "ancient road". ie. if you have a trade route, you get a free path through the terrain.
And then builders can build roads on terrain for a cost, and the cost increases depending on the terrain (and era). So a flat road is 20 gold, a road on a hill is 40 gold, forested hill is 60 gold, etc... When you get to a new era, your builder gets a new action for "upgrade road", so you don't get a free upgrade, but maybe just a small fee to explicitly upgrade the road to the next tier. But on the flipside, roads would also have a nuclear plant-like chance, where after 20 (or 40 or whatever) turns, it would fall into disrepair and act like the earlier road type, and you'd have to go upgrade it again. But similar to how traders build ancient roads, whatever they pass over gets a free maintenance. So your primary paths will never fall into disrepair, but if you have like a random path to a remote city that would need normal maintenance.
You'd obviously also need to add the "build road to", "upgrade road to", and "repair road to" options, so that you don't have to manually move builders constantly either.