Instead of +4 on the capital with Landed Elite, I could make it an empirewide surplus food bonus.
Maybe. I think it is worth testing the capital bonus first though.
I'm hesitant to give bonuses to the flatland building, since its main purpose is just to balance the 25% +1 bonus of hills.
Understood, but it is a weird way to balance it that feels funny both in terms of realism (why no workshop or windmill on a hill?] and because it comes in so late (it is late midgame).
Another alternative; maybe you can't build an aqueduct in a city on a hill? [How do you get the water to flow uphill?]
Something to point out is most policies are this way:
Not really. There are the defense buildings ones, which in general I don't like.
War is something inevitable, you are going to have to build military units every game. You are going to have trading posts every game. You are already have good incentives for wonders; the bonus is just a cherry on top, and the policy helps you to build them.
The defense buildings ones are different, because they're forcing you into building something that generally isn't worth building on its own, particularly in non-border cities.
Those can also be rare or absent from a game.
Not really, no.
Merchant navy is somewhat similar, but in a coastal city the coastal buildings were really worth building anyway. [Or at least they were, until you nerfed the harbors. What happened to the design principle of making coasts valuable? Now coasts are only 3 food 1 gold even with all structures built, and coastal cities are weak again. The Harbor and seaport nerfs really hurt!]