So the plan is to purchase a bevel sander soon. I'm mostly looking to just use it for cleaning out the holes for inserts and slightly widening a couple of them since they feel a bit tight compared to my other balls.

Is there anything I need to be wary of while using the machine? It seems pretty straight-forward but I'd rather ask and make sure before destroying a ball or two. Seems like all of the options for bevel sanders out there are pretty much the same models too so that takes away some decision making.

Learn how to use it with one hand so you can hold the vacuum.

Put it into the hole before starting it, especially with a fresh disk.

Like anything else, don't start on a good ball.

Hi - Bevel sanders do a great job for the intended purpose.  But it sounds to me that you need a bevel knife and some various grit wet sand paper.  You are fine tuning a section or two of a 360 degree thumb hole.   Using a bevel sander will open up the hole the entire 360 degree range.  Will make the thumb hole to large and quickly.  Is your issue at the top of the thumb hole or down into the thumb hole?  Be careful!  Coach Denny