aesthetic

For livable and more relaistic purposes, I say a joined aesthetic is almost impossible. Solarpunk has a lot of communal elements in it, so cultural differences would be a big deal, and also, as you say, enviroments can vary a lot, so even the widespread notion of greenery might not be practical in deserts or tundras.

Maybe the more general hints of solarpunk could be man grown plants and some form of green power source, even though it will vary depending on the region. As for the rest, I feel there are too many facets to cover, as you show in your postcards; abandoned cities, small mountain villages, desert plains, so many styles of farming...

As for your art, I would say it's a second stage inside of the movement. It's not the type of artwork you show people without context, I would do that with the more green utopia ones with blue sky; but when showing your postcards, I would also show their explanation, they need the context of the movement and of themselves. The first stage is for getting poeple interested with a nice view (and here common themes are more consolidated), but the second one is as personal (or communal) as any implementation of solarpunk features in real life.

(I'm not sure I made myself clear :$, just let me know if I haven't)

I believe the solution for that are symbols. As a really stupid example Hawaii looks rather different the NYC and different again to say Texas, but the US flag is the same everywhere and when you see a lot of them, you are propably in the US. So using the solarpunk flag is a good idea. It can also be a built or painted. That is true for other symbols as well. A cross usually means it is a church or some other christian building. The Communist have red stars and hammer and sickle. You could just use the solarpunk star instead. Slogans are another one and using the word solarpunk in some for might be a good idea. So:

In the mountain scene of a ropeway I did, I tried to reference steep-roofed mountain cabins and the kind of wooden visitor centers the state/state parks sometimes build around here for the transfer station building. In the solarpunk kitchen, I made the outside plain wood shingles on a fairly normal looking house. The one in-city scene I've done so far was a retrofitted parking garage turned mixed-use apartment building - that probably read the most visually as solarpunk, it was colorful, surrounded by plants, and the concrete had been painted with a mandala pattern.

I really like including art like murals in my scenes but they often act kind of like dazzle camouflage and confuse the scene, so simple patterns that are clear even when they're faint, seem to work best.

From the existing pictures I studied for the kitchen, dark wood and kind of traditional carved panels seemed to play a role in interior spaces.

Maybe cabin elements in cold places, steep sweeping roofs, big windows occasionally, especially to show plants or other solarpunk elements? Plants showing in greenhouses?

Cities, the mix of plants and grey buildings with splashes of color seems to be somewhat standard so far

I really like the two-layer idea of a few overarching visual themes and more specific biome-specific ones - I will keep thinking on it

They are possible, because modular designs are very efficient and sustainable, however, this being widespread is very unlikely. Nothing will be perfect, therefore it will need to change.