Why would you even consider an STM over, say, a multicore SOM, using USB or PCIe (and bridges, interfaces, FPGAs, etc. as needed) to concentrate data from the various peripherals?
Such a device also trivially(?) runs Linux, communicates over Ethernet, connects to the Internet, connects ordinary HID devices, HDMI or DVI or whatever monitors, etc. In short, a small PC with custom hardware attached.
Timing constraints for test equipment usually isn't terribly tight (i.e., on a similar scale as toggling relays), so the relatively poor timing of a multitasking OS is fine; real-time functionality is still available, with some difficulty (run as root? timing still subject to device drivers and hardware timing?), but preferably all hard real-time activity should be constrained to the peripherals themselves, and merely buffered data flows to the PC.
Tim