It feels more like a post-capitalist feudal system

We don’t have any indication of pre-jihad times in Frank works. Though both Brian version and Dune Encyclopedia point on kind of post-scarcity economy with huge role of AI. Brian also inserts the periods of Titan oligarchy and then Synchronised Worlds and Noble League between that and the Jihad. None of those were capitalist. So it wouldn’t be correct to call this “post capitalist”, rather the Empire was either the direct consequence of fall of the post-scarcity civilisation or direct continuation of Noble League (which was itself consequence of fall of the previous civilisation).

But still the rulers are descendants of a hyper capitalist class that came to own everything and with hyper-monopoly came rent seeking

Again, no info from Frank except Houses Corrino, Atreides and Harkonnen being directly descended from victors of Jihad - so not looking like the hypercapitalist but more like warlords. In Brian version empire aristocracy directly come from the Butlerian warlords that tore down whatever was before (look above) and League aristocracy. Only huge capitalist venture was VenKee corporation which was direct predecessor of Spacing Guild, but the Guild came to life as direct result of VenKee being smashed and transformed from a capitalist venture into the literal Guild.

I would argue something like the Spacing Guild to be an evolution of what Amazon could become if left unchecked to completely monopolize “the internet” and logistics.

Other way. Sure, that Amazon (VenKee) did aimed at domination over the empire but lost and was forcibly cut down to become Guild in nearly medieval sense of the word. Also had Norma Cenva as its real head who predicted a lot of future and was one of proponents of the new deal that is the Great Convention.