One challenge with the 4090 specifically is I don't believe there are any dual-slot variants out there, even my 4080 is advertised as a triple-slot card (and actually takes four because Zotac did something really, really annoying with the fan mounting)..you could liquid-cool and swap the brackets, but then you have the unenviable task of mounting sufficient radiators and support equipment (pump, res, etc) into a rackmount server. That assumes you're looking at something 2-3U, since you mentioned an R730; if you're willing to do a whitebox 4U build it's a lot more doable.

Of course if money is no object, ditch plans for the GeForce cards and get the sort of hardware that's made to live in 2U/3U boxes, i.e. current-gen Tesla (or Quadro, if you want display outputs for whatever reason). If money is an object, get last-gen Teslas. Tossed an old Tesla P100 (Pascal/10-series) into my Proxmox server to replace a 2060S with half the VRAM, for LLMs I didn't really notice an obvious performance decrease (i.e. still inferences faster than I can read), and in a rack server you won't even have to mess with custom shrouds for cooling, since the fans in the server are going to provide more than enough directed airflow.