Question How to make a smooth track (Splines)

Hi, I'm a somewhat intermediate Unity developer trying to make a first person runner type game where you stick to surfaces by changing the gravity and player rotation to align with the object you're standing on. One of the things I want to have is paths that corkscrew or flip around but the issue I'm running into is I can't seem to make a smooth enough one without things being jittery.


I initially tried probuilder and just kept making edgeloops but it didn't really help but I discovered splines sounded like they could be pretty useful however I am completely lost with splines and all I really want is to take a basic rectangle shape and be able to bend it around while still keeping it fairly smooth. I've heard people suggest using blender and then implementing it but that would be very very time consuming with how i'm playing to construct this game. I've messed around with splines a little bit and it seems like the best thing for me to use is spline extrusion but it doesn't seem to work when I try it with a cube mesh.

tl;dr how do I make smooth versions of the objects above using splines or is there a better easier method I've missed?