Sounds like you're using an older 126mm OLD freewheel hub with (possibly) a longer-than-normal axle in a frame with 135mm rear dropouts. Is this correct?
Unless you put all the spacers on the non-drive-side and re-dish the wheel, using a narrower freewheel hub with a larger OLD frame lengthens the unsupported part of the axle outboard of the bearings on the freewheel side, making it easier to bend or break an axle (as Andrew R Stewart described above). If you added spacers evenly to each side to keep the wheel centered, you did that.
f you feel you simply must continue to use that hub, then going to a solid axle may help. More metal (no channel for the QR skewer) means a solid axle would be somewhat stronger and stiffer. Downside is that you'd need to carry a wrench with you in case of flats, which would be much more of a pain to fix roadside.
IMO your best bet is to get a rear wheel with a freewheel hub and 135mm OLD. You could also rebuild the wheel with a 135mm QR freewheel hub, but that's possibly going to cost more than simply getting a new prebuilt rear wheel (new hub, very likely new spokes/nipples, labor if you don't do it yourself) .
Last edited by Hondo6; 11-28-22 at 08:37 AM.
Reason: Clarification.