What they want is a expandable pro machine like the iMac Pro without the screen. They want a taller Mac mini. They want something that you can build on. They aren't any less professional. They just have a different use case. The best Apple has to offer is a Mac Mini.
Let's make that clear. The next step up from Mac Mini is the Mac Pro. There is NO in between. That's a pretty big gap.
Please take a look at the SGI workstations of old.
A $6k computer does not deserve the ‘Pro’ moniker.
(I actually owned various SGI machines. They are very nice.)