You might want to peek at my reply in your other thread about the throttle/motor
https://endless-sphere.com/sphere/posts/1816198/
Regarding the trike, I also have a dog-carrier/cargo trike, built for St Bernards, so something similar would fit your Malanois easily enough.
The SB Cruiser has had several incremental versions / modificaitons over the years; older pics are first; after the pics is info about other aspects of your potential build.
Typical tadpole trikes mean you have the wheels under your knees and the cranks out front, in one variation or another. This makes it tough to have any cargo, especialy a wiggly one like a dog. If you put it up front, it's out past the wheels, unsupported, requiring greater mass in back to compensate especially during braking. If you put it in back, you have to stretch out the space between your seat and the single rear wheel, and that wheel must take most of the load of the cargo.
You can alter the trike to put the cargo between you and the front wheels instead, but that means you have to have *all* of you and the cranks, etc, behind the cargo area, and that increases the length of the whole trike more, and changes the way it has to steer, etc.
A delta is probably easier to build for a cargo setup, and is much easier to setup a front suspension (even rear suspension is easier). Depends on your DIY and design experience and skills for whether it's significant or not.
My SB Cruiser is used at 20MPH cruising speed and has no suspension, and works well enough for most of the road conditions--I would *like* suspension but as long as I avoid the worst bumps and holes, then it works fine. I use moped tires on the 20" rear wheels, but only because it was originally designed around a transaxle from a trike kit. If I ever get to build the mk2 version, it will be built like the below:
Regarding wheel size, as long as you keep all the wheels, drivetrain, etc outside the cargo area, you can use any wheelsize you want, with the cargo area (and you) as low as you like for the benefits of lower COG--If you setup the cargo area like my cargo trailer with the bed suspended between the wheels, frame hanging from the axles instead of sitting on them.. The cargo area (and you) can be anywhere over or between the wheels for best load distribution.
https://endless-sphere.com/sphere/threads/amberwolfs-flatbed-kennel-trailer-mk-iv.76539/page-2
The mk2 version of SBC would have the cargo area hanging from the rear axles instead, wheels outside the whole frame (even if they are chaindriven from a diff instead of hubmotors in the wheels). The trike I built for my brother uses this technique, but is only meant for very small cargoes. I don't know how well the specific trike would hold up or perform as he never bothered to use it other than a first ride or two, but the hanging-bed design works great for my big cargo trailer (biggest thing so far was a piano).
https://endless-sphere.com/sphere/threads/amberwolfs-raine-trike.80951/
If it's helpful, there are several other threads discussing various trike options for cargo and/or dogs if you look around the forum, that go into things in more detail, though I don't have any links directly to them.
These are a pics of the basic frame of the Raine Trike showing the rear frame that allows the cargo area to be lower than the wheel axles, so that larger better-riding wheels can be used and still have a low COG.
with the deck installed, and the bottom of the completely enclosed box the seat is mounted to the lid of (similar to the one on SBC)
A few pics of the various versions of the trailer showing the same thing
