"I used to load stls into tinkercad and export them back to my computer - and any stl you did that to would work 100% in openscad.
But tinkercad will no longer import stls."


I just checked and was able to easily import an STL into Tinkercad. I did just that last week to modify a Thingiverse file, with zero problems. I'm aware Fusion 360 changed some features on the free license, but tinkercad did its thing for me.