A broken spring is somewhat unusual but it does happen. Normally the springs are sized to match the projected GVWR of the trailer less the tongue or pin weight. If the spring shop people say that the springs are inadequate, they may be, but I doubt it. I suspect that you had a defective spring from the beginning and it took all those miles to destroy it.
You can consider heavier springs but you also have to consider the axles as well. If you have heavier springs, the axles take a greater pounding and will flex more and are more likely to bend out of alignment.It's only my opinion, but if you were able to go as far as you did on the original springs, then replacing them with similar sized springs would be your best bet. Replacing four spring sets is considerably cheaper than replacing an axle.
Edited by: Hamops
Helen & George VE3INB and Max (Bichon Frise)
2006 Silverado 2500HD D/A,
2006 Cruiser CF30SK