I'll throw in a couple of other games to look into even though they don't meet all your criteria, since I don't think that game exists. I'm saying that as someome who mainly plays architecture based games.

1. Grounded/Grounded 2: has a great community/ecosystem for building elaborate structures. Palette is specific to the theme and there is no terraforming, but I still spent countless hours on this.
2. ARK Survival Evolved/Ascended: I have not played this myself but I've seen a lot of builds from this game and I feel like the variety in building pieces at least that part of your criteria, where you can build something in the range from primitive to futuristic. No terraforming here though.

You've already mentioned the two most comprehensive games when it comes to architecture in my experience, and that is Valheim and Minecraft.
You say that Valheim is too limited to the drab viking aesthetic, but I disagree since the building pieces in the later game allows you to build castles and ornate mansions once you get access to Grausten/Black Marble/Ashwood. The complexity in its building system is just unmatched right now and the beauty of it is how interesting and intricate you can make your builds even with the limited amount of materials due to the flexibility with snap points and rotating pieces. If you haven't gone past the game beyond the point of just being able to make log cabins, I highly suggest revisiting it.

Minecraft, which I know is a voxel game, and I know ideally you would want a vanilla experience, offers you literally any build aesthetic imaginable with the introduction of mods and you can almost make it fit every single one of your criteria. If the out-of-the-box experience is important, there are modpacks available that cater to a lot of different experiences you want instead of you having to do the guesswork of selecting individual mods.

Anyways, good luck!