Rufus is interesting indeed - presumably also pretty unusual, although it would be more interesting to know just how unusual!
Have you seen Kennedy's article
C. Velius Rufus (Britannia, 14, 1983)? Kennedy proposes a slightly different chronology to the one you follow (Dobson's?), with the Carthage command and Mauretanian war happening before the vexillation command, which he suggests was the same force used in the expedition across Dacia rather than in an earlier German conflict.
Wasn't there also a correspondent of Fronto called Velius Rufus? He might seem a more obvious choice for the retiree mentioned in
Meditations. Although probably a son (or grandson?) of the military man.