The weak link in the "keep the 8-3/4" argument is that it's worth quite a bit of money now, as-is. What it's worth after it breaks is obviously much less. The difference between the 2 makes the new replacement (whatever) that much more expensive.

I'm building an off-brand mongrel, since it's a Fox the 8.8 is an obvious choice, and 31 spline axles with big tubes and discs are all over the place.

BTW: I don't know how the concept of rating axles by their spline count started, but obviously a 35 spline axle is not the same as a 30 spline axle with different splines.
If the spline contour is universal, axles can be better compared for strength by assuming the diameter before the spline is proportionate to the spline count, yes (not completely accurate since the splined area is a bit larger, but close enough)?
If so, compare axles (ceteris paribus) using the cube of the spline count per the Young's Modulus vs. diameter relationship.
If axle #1 is 30 spline and axle #2 is 35:
35^3 / 30^3 = +58.8% in torsional resistance.