getting the ground perfectly flat in a large area is between frustrating and unpossible. There is an element to valheim that triggers OCD too -- a lot of pieces, even when snapped, are not exactly 100% aligned visually. It works, and it looks pretty good in their style, but I think some folks can't get past it. Same goes for level ground... you can work without it by simply putting down a floor. A stone floor will counter up to 1 meter of unevenness, which is a lot by valheim standards. A wooden floor with the occasional beam under it will also counter a good bit of unevenness. Between those and raise/lower/hoe terrain stuff you can get it to an acceptable place, for most people.

as for the other... Ive made more than 10 floor buildings without mods/hacks/cheating. Its no trouble to do that, if you need help, ask?