I’m modifying the tow rig portion on the motorcycle to be fixed and rigid to the moto frame and the hitch and swivel point to be at the very rear only
The ever-dreaded can of worms... When you accomplish A, then it impacts B which must then be addressed. Building a swivel and U-joint at the rear of the bike completely eliminates the unlikely but possible side torque on the bike. But, now, all the trailer weight is behind the bike. The rear shock preload
will have to be adjusted or the shock replaced to compensate for the weight. This would be a great time to consider a progressive (dual rate spring) rear shock but one must consider the fulcrum affect which will reduce weight on the front wheel as a direct correlation to tongue weight. Stiffening the rear shock
will cause less weight transfer to the front wheel.
I have a travel trailer with heavy tongue weight. Even with load levelers my front wheels wear unevenly as a result of a fulcrum affect of the weight on the hitch loading the rear axle and reducing weight from the front. Adding spring strength to the rear axle directly reduces weight even further on the front axle so with this trailer fully loaded, the front axles weight is less than when the truck isn't loaded.
Many years ago, I commissioned a trailer for my motorcycle that was small enough to be towed behind my bike yet strong enough to carry a motorcycle. It was indeed small enough, and strong enough but way too heavy. My front wheel would leave the ground on acceleration and I was never able to tow it with a motorcycle. I still use it to carry stuff but behind a vehicle. Tongue weight on Blitz's Shadow won't transfer to the frame anymore and must be carefully considered when loading your trailer to keep enough weight on your front tire. Your initial hitch design shared the load between the front and rear wheels.
Additionally, the height of the hitch is a critical dimension now as well. If your hitch is above wheel axle height, you'll be more likely to wheelie during acceleration. If the hitch is below the axles, your front wheel will get lighter during braking. That is regardless of tongue weight. So, elevation is a consideration. IMHO, you'd want your hitch to be slightly above the axles so your front wheel will be loaded during braking. But not so much that your front gets overloaded in hard braking. Once again, your initial hitch isn't affected by this either.
So, basically, you'll be trading one potential handling hazard when in unusual angles to another potential hazard caused by cargo loading.