There is a mod that makes floors act like roofs (Floors are Roofs). Does not add a flat variant of thatched, but essentially does the same thing. combined with Gizmo Reloaded, or the advanced editing in Valheim Plus you don't really need it. Floors can look just fine as roofs. The next major game update though is supposed to add more roof variants. I suspect Hearth and Home will break half the mods I use. 8(
Hearth and Home does look good though.
https://youtu.be/hY2z7AkxrBw