If all else fails, use the hoe! Sadly that's only partially a joke. The closer a structure is to ground the more stable it is, general rule...that includes if you happen to bring the ground up to it as well. There's a max height you can raise or lower terrain from it's original height though, and if it's at it's max you aren't going to have the wiggle room to flatten it out and make it a bit prettier, but it is effective. Also worth noting this also applies to already high terrain or...set pieces i guess? Not sure what exactly you'd call things like the giant ancient trees or the rock spires you can also build off of. Bonus that earth walls also make effective base defenses (be aware some enemies have a pretty extensive reach and others fly, so it's not fool-proof but it helps in like 90% of cases).

It's also possible to raise terrain on (technically next to,) already built pieces without moving the pieces. They will just sink into the ground or float respective of whether you are adding or lowering terrain height (floating pieces will update their new structural integrity and may break in the process). I personally do enjoy doing this with my outer walls and the large stone pieces, partly because i also tend to put my pathways (preferably roofed, which i often build off of those stone walls built into the earth wall) around the inner edges of my outer walls to give it a bit more buffer space for when i do get a troll raid or something. Nothing that important or difficult to replace ever gets damaged (except maybe a buried ward i have to dig up to fix, not hard, just have to remember where the heck it was for spawn blocking reasons).