You can run UCCNC in a virtual demo mode (ie without a physical interface to any machine).

Mesa? Depends on your use case, I use a 7i76e to control three steppers and an analogue vfd... vfd is being replaced with a servo and the card is running with that in step/dir position control, but I’ve yet to integrate the servo encoder. Two hand encoders (low resolution), and more I/o than I can shake a stick at.

The hard sell is recommending LinuxCNC, no way as near polished as the pc software, but offered as you said non-pc based. Would recommend eyeballing that a long time before even considering buying the motion controller, as you might write it off immediately.