Here’s an idea to help alleviate the “slope too steep” issue when trying to build. I think the mechanic is perfectly fine. However, it seems too strict and forces the player into only one action: Flatten Tool.
The Flatten Tool is pretty labor intensive. I propose to alleviate strictness of slopes to choose an option increase material cost of buildings based on the amount of slope you’re trying to build on up to a point where it is still impossible without flattening. This allows players to do cost/benefit if they’d like to pay more material cost or spend the labor time to flatten the land.
If you build on slopes you may need more foundation bricks, stilts, etc… or just general land prep which cost materials. Higher material cost also increase total time to build since it will require transporting more building materials to the site.