Marius' reforms, while creating an even better professional army, effectively destroyed that deep reserve and imposed an enormous burden on the state in terms of pay. The Emperors, through various confiscations, had acquired enormous properties throughout Italy and the Empire whose revenues (along with booty) were sufficient to pay the army in the early Imperial period.
Creating new legions and paying them became a problem already by Marcus Aurelius' reign when it took 20 years to beat back the Marcomanni. Returning to a citizen army was too much of a threat, as the Senatorial class would then have soldiers to challenge the Emperor. Caracalla made all free men citizens, which eased the recruiting shortage but not the problem of payment. In the end, the old Senatorial government was too unstable to govern a vast Empire but so was the Imperial government, at least not without reducing the effectiveness of the mass of the army on the frontiers, which left the Empire prey to foreign adversaries. It was an even worse problem in the West because it was lower in both population and wealth.
After the defeat of Antony, Augustus cut the Roman army from 56 legions down to 28 (some had been understrength anyway), created the Praetorian Guard for city police force duties and imperial bodyguard work, and created 28 auxiliary legions from among the conquered people.
There was already the issue of a basically fixed Roman population (even after bringing in the ethnic groups of Italy) having to occupy, police, and protect provincial populations and borders, so by Marcus Aurelius’ time the regular legions were drawn from non-Roman groups who (mostly) spoke Latin (or could get by).
The Empire was also struck by a plague. An analogous scourge (probably the same bug) hit China. Unless the barbarians were the carriers and had already worked up a tolerance for it, the plague probably saved the Empire. It emerged subsequent to the earlier war with Parthia, which lost, and was probably struck by the plague at or just before that time.
The border problems probably stemmed from a periodic pulse of Central Asian ethnic groups who spread out in at least three directions on a regular basis, centuries apart, corresponding to natural climate variation. After the Romans’ defeat of the Sarmatians, their equestrian culture resulted in a handy source of auxiliary cavalry, which was moved to Roman Britain. The locals didn’t like them, leaving them beholden to their Roman supervisors. That’s an old tactic, common in ancient empires.
An unintended consequence of Trajan’s long war of conquest of Dacia was the absence of what could have been a buffer state between the Empire and the wild wooly wilderness filled with who knows who. Trajan’s last conquest extended Roman rule to the shores of the Persian gulf. Hadrian was no warrior, and he wanted to kick back and knock off young boys. Roman military decline began with him.