I don’t know about cesium or any of that “real” tiles stuff, bur a spline is just a set of points.
You can make an excel sheet with all the points, and import the spline.
This is normally done for stuff. Since for instance i have 3000 rivers/brooks and 7 major roads with around 1000 little ways to import in a 144km^2 gameplay area.
All you have to do is figure out a format, get a data source, and create a bluetility that will process that source and generate your spline for you.
Some times you can select the whole spline points and hit home to make them find the ground.
Wouldnt reccomend doing that on splines exceeding 100 points though.
And import wise, you want to avoid long loops or the engine times then out.
Build it so you plot 10 points at a time, then do something else (like add a random editor only mesh? Im using mile markers.) and plot more.