Looks like quite the set up you have there for your hobby! Very nice.
Hopefully someone pops up - if not already - to create your terrain. And if not, try investigating in the model railroad world though you might have to educate whomever you find on your needs - but there are definitely commission modelers in that world but comparatively more expensive which doesn't seem to be an issue for you.
If you end up having to "do it yourself" I STRONGLY recommend you use extruded foam with some sort of batting around the outer edges and rigid basing underneath which will give the pieces strength and durability - and maybe embed some magnets and steel plates (thin and small) to help hold the boards together when in use. This also allows for sunken areas in the terrain. And in your commission, perhaps ask for some features that match that can be placed on top of the boards to provide elevation, hills, mesas, etc. And keep playability firmly in mind which I'm guessing your years of gaming has already taught you. Terrain can look spectacular and very realistic but playability requires significant compromises.
Impossible to know if I was first, but I was the first person I am aware of to create realistic looking undulating terrain (not counting sand tables) back in the 1980s, everything else was flat as a pancake with blankets and books and lichen predominating. It was my model railroading background I suspect. It looked grand for its time but it proved impractical in the long term because figures wouldn't stand well. Why I now use "wedding cake" hills.
Again, good luck on your project.