>>> The split for a multi-open door though...man, that'd be so cool. If you have any resources, please share em!
Realize first that the aluminum angles, tubes and flat bars available at your local hardware store are really easy to work with. Joining parts can be done buy overlapping flat bar or cutting short sections of angle to make direction changes. It does not need to be welded either if you use a really good rivet like these
https://www.mcmaster.com/98778A541/ and if possible, a layer of VHB tape between the joint.
I'm the type of guy who figures if I want to get something, I just make up my mind I'm going to do it, and I'll figure it out as I go. I also am fully aware of my favorite reminder, that if I "keep throwing money at it"... Well, I'll get it done eventually.
That said, If I wanted a 3 Door, I'd pick a dimension of where I want that top section to end, and I'd cut clear across that door from the back side with a circle saw, interior wood and all. Then I would pull the wood, and carefully rivet a piece of angle across the inside about 6" higher than the cut, just to hold things in shape.
Now with the "door" lighter, I would get the top hinge system working. For ease and a leak free environment, I'd spend the money right away on a hurricane style continuous hinge like used on your typical teardrop build uses. Wouldn't take much to get this far !
Then I would attach a horizontal aluminum angle across the bottom of the new cut edge. I'd position it so the "L" shape has a leg down towards the back of the trailer. This leg would serve as a closure strip across the entire back sealing the top door edges when they are closed. SO it would need to remain towards the rear enough to clear any future trim that will exist on the top of the doors and perhaps the weatherstripping I'd put across it. Remember the lower cut off portion will become doors, and the top edge of that is currently a raw edge.
The above angle would be riveted to the lower portion of the upper half by using 1/4" aluminum angle cut to be brackets to allow it all to be riveted together.
I'm not sure what type of hinges your original ramp had at the bottom, but I would leave that lower section in place and locate the hinges you want to use for the two swing doors. I would assume that the same ones used on most side doors would work fine. I'd flip up that lower ramp portion and fit/mount those hinges next. Once they are in place, I'd rip thru that lower portion exactly where I want my doors to split.
I'd rebuild a new door jamb system where they would meet in similar fashion as the aluminum along the bottom edge of the top portion. Note too, that if for some reason you are not comfortable with just the riveting, you can always remove any of the much smaller sections and take it to a weld shop where a few beads of Tig can make all the difference in the world. At least with the rivets, you have everything where it needs to be.
Latching should be pretty straight forward once you see what you have to work with. There is no shortage of really good door hardware from places like Mill Supply who has hardware for Step Vans.
I'd love to take on such a project... Not something I would fear at all... Well, other than costs these days. OMG. I bought my Cargo in 2018 and attacked the conversion. Thankful that I did it then because today, I don't think I could afford to do it ! Stuff has really gotten out of hand and sadly I don't think it will ever really come down to where it was.