Is it not so that there was a point at which Roman soldiers, who were drawn entirely from land-owning citizenry, had the empire eventually caught between the need for concessions to non-Roman Italians and the cause of the populares?

The early Populares, when the land-owning citizenry were still nominally a concern for military manpower, were in favor of concessions to non-Roman Italians. The later Populares existed in a scenario where the Roman Legions freely enlisted landless citizens, and the concessions to non-Roman Italians had already been made and were no longer a major concern.

All the land being owned by great estates, the equestrian class gaining increasing economic power due to optimates not being interested in commerce, and all the trouble that was caused once the Gracchi brothers demonstrated the true power of using the tribune position in ways not anticipated by political norms?

The first issue, though a genuine concern, is also often overemphasized. Freeman farmers remained in control of a large proportion of Italian land, and arguably the bigger actual problem was the decline in usage of paid tenant labor by the great estates due to the influx of slaves from successful wars in the 2nd century BCE, which led directly to a glut in the slave supply relative to the citizen population that would be literally unequalled in Roman history.

The second issue is not really core to the Late Republic's problems, and certainly not the Empire's. The Senatorial class was very interested in commerce, but legally barred from directly participating in large-scale commerce. The loophole was to use the Equestrian class as their agents and reaping a portion of the profits. Many of the more sensible reforms involved bringing the Equestrian class into a greater share of power in the Republic at the expense of the Senatorial class - something many conservative Senators wished to avoid. There was no sense in giving the Equestrians more bargaining power, after all!

The third, I would argue, was an effect more than a cause.

I confess I only have a few overview courses under my belt. My point was only that the late republic’s interests abroad became a source of material disparity that was likely to collapse eventually - drawing a crude and/or sly comparison to current events.

Ah, I see. I thought you meant the entirely of the history of the Roman polity when you said 'Empire', rather than the fall of the Republic/that phase of the Republic (hence me bringing up the Late Empire).

In that case, I'm in agreement - the concentrated wealth caused by the Republic's expansion exacerbated class conflicts to a degree that the institutions of the Republic were simply not made to handle.

When I made references to democracy, I was speaking only of the fact that there was voting being done - even though there was, I understand, an amount of flaws and limitations to democratic expression that we would not find inspiring even though our own systems are so badly flawed. I did call them pretenses of democracy.

My point here, though, isn't denigration of the democratic features of the Republic (meagre though they were), but rather pointing out that the norms violated by the Populares were very often leveraging the nominal and formal strength of what democratic powers there were against unwritten norms that those democratic powers were not to be used against the Senatorial aristocracy's interests or consent.

Democratic laws (or processes) against anti-democratic norms, if you will.