I don't think the roads should take longer to build. 1, 2, and 3 full years is a substantial amount of time. The Romans built the Appian Way in only a year or so.
Your points about the farms is well-taken. I think the problem here is that we're trying to represent two separate things with that tree: first, the clearing of the land, and second, the development of agribusiness. I think the latter can be safely represented with a relatively short build-time (especially because it is tied to settlement size, so some of the social changes that are prerequisites for the different farms have been happening in the background for many turns), but you're right that we haven't considered the form sufficiently. Therefore, does it seem reasonable to rebalance farm build times from
4
8
12
16
20
to
20
40
8
12
16
The time increase will be counterbalanced by a massive taxable income bonus from farms, making them much more strategically valuable than they are now (farming was pretty important back then, but isn't currently in-game).
The income bonus will be balanced (and then some) by increasing all upkeeps by a factor of at least 1.5. Probably two, because I'm lazy and want all ExRM players to suffer (including the AIs--I don't discriminate).