After posting this question to Physics, it became pretty clear I should have posted here. So:
How might a (e.g.) 72-bit crypto-relevant quantum computer attack RSA-2048?
Bonus: how might that be characterized? (e.g., nn-qubit requires xxx passses, run time ~yyy)
Shor's algorithm appears to allow for parallel execution or iterative runs with a combination step. Assumption is that smaller-qubit QC might be able to perform those pieces.
However, it is suggested that a 4000-qubit/100m-gate quantum computer would be necessary. As the quantum piece of Shor's is a large transform, I assume that sets the constraint for qubit-size
Side note: there also appear to be possible speedups that may reduce the run time, such as qubit recycling? or the 4-8 passes vs. the 20-30 passes (by David McAnally)