During its run, the Empire generally relied on hired guns to collect taxes, who were, uh, very thorough, because they got a cut, rather than have a bunch of undermotivated lifetime corrupt bureaucrats.
Diocletian ended the empire's Crisis of the 3rd Century, required supplicants to conduct their audience face down with their arms straight over the direction of their heads (this is where the Popes picked up that practice afaik), divided the empire into four parts, set up the first system of succession, then put it to the test by retiring and compelling his eastern colleague into stepping down at the same time.
Lapses into seeing internal rivalries as the more important threat, at bad moments when the external threats were much more serious, turned out to be the biggest problem faced during all of Roman history.
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.