For more comparisons:

Vespasian (Pagan, Principate era, ~70 AD)

Caracalla (Pagan, Crisis of the Third Century, ~210 AD)

Decius (Pagan, Crisis of the Third Century, ~250 AD - note that, despite being only ~35 years away from Diocletian, the facial features here are very rough and ‘realistic’)

Diocletian (Pagan, Late Empire, ~284 AD - note the departure in style - smooth face, large eyes)

Constantine (Christian, Late Empire, ~320 AD - very idealized, almost wiped clean of features)

Valentinian III (Christian, Late Empire, ~455 AD - idealized into abstractness, the ‘idea’ of the man who is Emperor)

Oh, and one for the road - a portrait of a NON-Emperor, 5th century AD - note that though the style has hints of the Christian Emperors, it has far ‘rougher’ (and IMO, more beautiful) detailed features - because it’s not trying to be the ‘idea’ of the Divinely Ordained Leader. It wasn’t a loss of talent (that doesn’t come 'til later, when the entire Empire collapses in on itself) - it was a weird stylistic choice.