There are some other things to consider. At the end of the day, there is no one size fits all best solution.
First, volumetric restrictions are very important and may present critical constraints on whatever solution you might fancy.
The best payload arrangement (ie weight) is a stripped cabin and bulk loading. Penalty is time to load and unload and the maximum package weight/size which can be manhandled into and out of the cabin.
If you want to go for conveyors, that's fine, but are you going to have them at all ports ? There is a capital and ongoing maintenance cost as well as time and labour costs for an each time set up and removal. A cost/benefit analysis will yield an answer to the question of whether they might be a good idea for particular circumstances.
You can design and manufacture suitable pallets to suit. I did that, decades ago, for several Bristol Frighteners. Relatively low cost, not too much weight added to the cabin and improved loading/unloading times significantly.
You can go the certificated ULD route but you then have to pick from the standard can sizes and there is an empty weight penalty for the aircraft mods if you are looking at integral restraint. The best setup I ever worked with was the roll-on roll-off arrangement IPEC used on their Argosies, now long gone. See, for example,
AWA ARGOSY (chota.me.uk) seventh photo from the top. Doors front and back - push the inbound cans off one end as those for the next leg come in at the other. Worked an absolute treat.
Certainly, not a simple matter overall but there are options.