What program are you using to make the models in the first place?
When I bring over models I’ve made in Tinkercad, no matter how simple and straightforward the model is, it gets imported as an utter mess. What should be a straightforward, no-frills cylinder winds up looking like a crushed paper towel roll that someone’s tried to reshape back to normal. What should be a flat surface is actually a little bit skewed. The list goes on. There’s just something nutty in how Tinkercad writes its STL files, basically favoring speed over fidelity, likely since they expect people will go straight to their slicer program (which is a little more forgiving when it comes to lost details).
Meanwhile, someone gave me a model they made in Inventor and the geometry was a thing of beauty. There were no gaps, all the curves were evenly-spaced out, flat surfaces were actually flat… it moved me to tears.
If you’re importing models other people have made from someplace like Thingiverse, that’s a bit of a crapshoot. OpenSCAD models tend to look pretty good, since the setup is more barebones sort of affair.