I have been a cabinet maker for over 30 years, in that time I have never been able to SAND anything square or flat, especially with power sanding. I assume it's physics, the part of the sand paper nearest the power turns faster than the parts further away. Presumably there is a "technique" to overcome this (i.e., moving item fast across the sander, alternating pressure, etc) but I have never mastered it. So I avoid power sanding anything for square. Professional miter techniques for picture frames involve a small 45 shaving cut from a trimmer (never sand, tight square joinery has always been based on cuts). A barrel trimmer or mill is the closest to this "cutting" approach ---- but they are expensive and require sharpening or replacing after too many uses. My suggestion:
- get a good jig to re-cut your blanks after gluing barrel (band saw or miter saw)
- cut them very close to barrel
- use mill/trimmer in a drill press (because you are only trimming small amounts of wood the cutter last longer
- after turning- if needed final squaring by hand sanding with a jig that uses little discs of and paper on a mandrel that keeps your tube square to the sanding disc (some guy on here sells these jigs at a reasonable price)
If you are still set on a power sander - there is a commercial jig - I believe it would work best dead center on the disc - which I think means you will go through the center area of those expensive 12" sand discs - and I agree not much difference in any of the combo bench top sanders - probably better with a 12" disc BUT the most important part of the equation is if the sanding table can be squared to the disc and is big enough to use that commercial jig - my 6" combo sander table wasn't big enough for my squaring jig