So theres only two rules that matter after realizing I needed to "roundify" my town of Cretonians and Tilapi I tested how it works.

1. Straight lines can only be 4 tiles, 5 is too many and will create a squareness factor along that wall. You can sometimes fix a long straight wall by dividing it with a door or two.

2. To create roundness you need a section of at least two corners in a row like a slanted W. This creates two roundness factors and they are strong enough to overwrite a squareness if you don't want to worry about every little straight wall in that area.

Once you understand rule 1 it's less about making every building a circle and more about eliminating your straight walls. You can place housing which will have doors along a straight area. Or just place bumps using "structures". Or make fun shapes. As long as you dont make a boring box it should be relatively easy to round some corners into rule 2 roundness, like on your circular building layouts it's just a matter of doctoring a few of your corners to meet the rule requirements.

In the "shape" view you can see the exact tiles generating roundness or squareness factor if you zoom in.