If it needs a new axle it’s on me. I didn’t have issues with the Sullivan and only through the Irving like twice. I through the pants off that Sullivan came apart a bunch of times in the beginning but never snapped a string.
Cotton 10 is all I used. Maybe pull a “burn in” I think that’s what Ed calls it but will search. Although I did that first thing what I got the Sullivan. Is it snapping at the loop or higher? If it’s higher than maybe the axle needs to be more flush.
I had the same thing happen with my new Morrow. Turns out one of the axle threads, which are very sharp, was exposed just enough to cut the string. After going through four strings I took it apart and reassembled it taking extra care to make sure everything lined up flush. No problems after that.
Found the instructions, start with no string on the axle, wrap the string (cotton) around axle once and slowly pull it so it rubs the axle. Also I’d wax it, bees not chapstick.
That certainly won’t be needed either way man. That’s an extremely kind thing to do, but especially since it wasn’t giving you any trouble I’m sure the problem is somewhere on my end.
In my experience you just need to ensure that the wall of the axle is flush with the wall of the yoyo. After that, I do “prep” axles by basically wasting a cotton string to make sure they are free of any hidden burrs. Really haven’t had an issue with snapping following that algorithm with any yoyo. Granted lately I haven’t been throwing for distance lol.