First, much of the expansion of Rome happened during the mid-Republic, in which the customary/traditional economy predominated, so don’t put too much stock in capitalism or protocapitalism as a reason for Rome’s expansion. Rome, ultimately, was an incredibly aggressive, proud, and strangely assimilationist (for the time) polity, so it had the relentless warring with its neighbors to maintain an experienced and well-tested military system, the arrogance to take over everyone and anyone who seemed ripe for it (I would like to note that the arrogance of a Classical republic is a funny and dangerous thing, because civic pride is much more sustainable than the ambitions of autocrats), and the ability to both use their new human resources not simply as slaves, but also as members of the polity (and thus, both assistants in further expansion and, in the long-term, stable and loyal populations in their own right - to varying degrees),

Second, the Roman Empire’s financial customs were, by modern standards, primitive, but by Classical standards incredibly advanced. They effectively united the Mediterranean into a single market, or at least a series of closely connected markets, complete with mobility of labor and capital. Crude forms of joint-stock companies and corporations had legal standing, contract enforcement was rigorous, and while credit was tracked on an informal basis, moneylending (from both individuals ad hoc and dedicated professionals/businesses) and debt were essential parts of the overall functioning of the economy. Roman aristocrats largely held a very rationalist financial-oriented mindset, unlike, say, Medieval nobility or Chinese bureaucrat-scholars. They were quite literally investing capital and managing businesses for (sometimes grotesque) profit. So while you can certainly make arguments about the relative balance of the traditional economy to these very advanced parts, on the whole, I think it’s very fair to regard the Roman Empire as capitalist or at least some form of protocapitalist, like the Netherlands in the 15th-16th century, just making the transition.

Third, the Roman Empire’s capitalism or protocapitalism definitely pushed forward economic development, if not necessarily expansion. Some recent work suggests an average GDP growth rate of 0.1%-0.2% per year through the first 200 years of the Empire - incredibly sluggish by modern standards, but an unheard-of sustained rate of growth for pre-modern polities that would not be matched until the Netherlands and England in the 17th century AD. It would be hard to argue that this was technological as, for all the losses experienced after the fall of the Empire, technology in Europe did continue to improve after its fall. It seems pretty intuitive to attribute this not to technological advancement, but economic organization - ie the development of a capitalist or protocapitalist mode of production.

Fourth, independent funding of armies was a very temporary thing. That was only really major in the Late Republic, and it caused a lot of civil wars. Furthermore, individuals could not raise an army nor make war without consent of the Senate (at least not in their capacity as Roman citizens), and command was only granted by appointment of political office - typically by the Senate (or by the popular assemblies). So there wasn’t quite that level of independence in plundering and looting the surrounding states - Rome liked to have a pretext for its wars, and generally frowned on large private forces. Caesar himself, for example, only was in command of forces to begin with because he had been granted command of a province (which he lobbied hard for, mind), and then had to get special permission from the Senate to raise additional Legions (which he himself had to fund) when he started his conquest of Gaul (under the pretext that a Roman ally had been attacked and he HAD to intervene - to save the allied Gauls, of course!). His level of independence in starting such a massive conquest was only rendered possible because he was one of the most politically influential men in the Republic at the time (in an alliance with two other incredibly powerful politicians) - and even then, despite having one of the most glorious and republic-expanding/enriching conquests in Roman history, almost got him arrested and executed for illegal warfare at the end of it all (the suspicion of which kicked off the civil war which Caesar eventually won). So even that lower level of independent conquest was far from the norm.

Fifth, Bread and Circuses is a bit of a funny thing. The initial grain dole was part of an overall reformer effort in the Late Republic to restore prosperity to the (at the time) hard-pressed working classes… by returning them the power of a traditional economy in which the majority of citizens would engage in subsistence farming. By the time of the Empire, the importance of bread and circuses was political - less about the economy, more about the fact that, for the 1st century AD, into the 2nd, and arguably into the 3rd, the population of the city of Rome was still aware of its informal political power (however reduced), and could make and break imperial officials, up to the Emperor himself in particularly dire cases. So there was a definite urge to keep the city of Rome as placated as possible.