My dirt is red clay with tons of rocks. I only work 4 acres and it works great towing with a ATV. This is not my original idea and it was plan B since I didn't have a welder. I am 67 and saw farmers when I was a kid rebuild packers with wood. The older packers were all bolted together but the newer packers were starting to be welded and welds failed just like you are experiencing. One old farmer told me the wood could flex whereas the metal welds fatigued due to vibration of the rocks. Some even used railroad cross ties for the axle mounts and being impregnated with creosote they lasted forever and were heavy and cheap. The best packers back them were the old horse drawn packers, built to last two lifetimes. They werent pretty like todays painted packers but boy would they pack dirt, I towed one many miles with an old Minneapolis Moline.
the packer I posted a pic of is over 50 yrs old and never a broken weld on it to date!, but the original owner and ME< never abused it, we stuck to going slow and steady with it, which I think makes a huge difference
but the fact you say metal one's flex and such and cause welds to break, that is what I see being the problem with wooden one's
I am not far from a LOT of Amish farmers and I never see them using wood ones, due to that wobbling and such tends to make the wood egg out and nails and fastners pull out
NOT trying to bash on your build to info, just saying what I have seen and know about
the main farm I managed for yrs was a 100+ yr old farm, started out with all horse drawn implements(many still on farm) and all the wooden one's have come apart , same with the wagons too!
but its very un even ground here and a LOT of BIGGER rocks, over 15 yr period of bring in a rock picker every other yr or so, pulled about 125 tri alxe loads of roaks, filled a 5 acre field with dumped loads of rocks, I swear tha one field grows rocks better than crops you can sell LOL
BUT then again I did sell almost half of all the stones, so they did make a couple bucks back, but not enough to pay for the rock picker yet LOL
and there are many rocks that the D 6 dozer I had couldn;t even make em twitch, they are just one's you have to go around now!