I may be misremembering but I think Pikmin1211 had a good succinct (10 minutes?) explanation of map structure and design. There were a couple finer points which you could debate or argue either way (He’s more strict about “dead space” than I am), but it’s a great starting point/frame of reference if you manage to find it.
I think the reason for that is it’s sort of like: “How do I write a movie screenplay.” There are several pointers and rules you can follow, but the exact flow, pacing, structure, and lines are all up for the writer to decide.
As for the graphic side.
You’re right, nothing exists for that yet. For the time being, you should be able to ask for map feedback in the discord server. A general rule of thumb I’ve found is that for every main map piece (mountain, forest, house, road, river, ect.) there are auxiliary tiles which blend that piece into the surrounding area. By placing “Dirt-Grass” next to a house instead of “Grass-Grass” you’re able to increase the visual quality of your maps significantly.
Another very general rule of thumb
(again, likely not what you’re looking for) is does your map look “Lived in.” Unless you are in the absolute wilderness, each map is going to have some kind of human element. For example it makes no sense to have a fortress with grass stretching out in all 4 directions. There should be a road coming out from 1 side and logically people would not build a fortress in a plains unless it’s a key strategic location or it is a country manor (In which case it should still have a road.)
It’s one of those areas where by spending time with maps that you like and don’t like, you begin to figure out what makes for good map design.