the op says after calculations he has plenty of weight left
and i did say that i had concerns about what looks like a home made bracket
in my opinion its fine having a home made bracket if it is strong enough
this is why i ended up making my own bike frame as i just did not like the flimsy stuff that is about
i do agree with what you say and you are right about the loading and i hate to think what load would be on the bracket when it goes over some bumps
this is why i asked for some information on the materials used and as i said it did not look strong enough to me
680k spare tends to suggests that’s total payload allowance, no mention of axle loading. Once you start moving rearwards from the back axle the leverage effect of any extra weight adds weight to the rear axle VERY quickly (and disproportionately)
Still wouldn’t risk that loading on a towbar I knew nothing about, far too risky.