The cultural and legal institutions of feudal Europe were very hard to breakdown, and in places like Russia didn't start falling apart until the 1800s, well into modernity.

In many places, those cultural and legal institutions still exist. That's the point. The cultural, legal, social, economic and political foundation of the modern day weren't laid in some 'capitalist revolution', they were laid over thousands of years. The events that you cite- the Black Death, the Renaissance etc.. were not 'revolutions', but slow turning points along a spectrum, which contrasts greatly with Marx's historical materialism, in which history may be split into clear stages punctuated by radical social change.

No institution went untouched.

It depends on what you mean. Trading, employment, banking, finance, monarchy, the church and so on all existed before and after. It is true that the role of each institution changed over time (although there remain theocracies and absolute monarchies in 'modern' countries today, of course), but this does not require total social revolution in any sense - it is unlikely any institution would remain absolutely immutable for thousands of years.

but I'm willing to go out on a limb and say you're wrong about that as well simply because you're framing a society that exists farther away from the Roman Republic in time as we are from the Roman Republic in modern terms.

Economics as a field did not exist until the 18th century, so any discussion of the 'economy' of early civilisations will necessary be framed in the language of today. As for ancient Egypt, it has the hallmarks of a proto-capitalist (or 'commercial capitalist') society, albeit one with heavy state control.

It'd be like of I called every hunter-gatherer that existed proto-communist

Karl Marx did just that. It's expanded upon in "Origins of Family" iirc.