>follow on decoupling from Russia
You are aware the deal was entirely dependent on Russia, and the follow-on Biden wanted to sign (but couldn't since Iran wouldn't fully cooperate with IAEA) involved Russia even more heavily? There's no other place that both sides accept can store the enriched Uranium or supply fuel rods to Iran.
>Isotope separation by centrifuge as a physical process follows the laws of diminishing returns
It's the other way around. Going from 3 to 20 percent is much harder than 20 to 60 which is harder than 60 to 90. Going to 99.9999% would be tough, but is unnecessary even for nukes.
>"Just a step away" was more a hard bridge to cross back when third party inspectors were at the enrichment centres
They were allowed to enrich to that level under the deal starting in 2031, inspections would have tested if the enriched material was diverted.
Even if they could be effective at such short notice, it would have taken the US being distracted by some other crisis and being unable to act in the short period between detection and weaponization to lead to a nuke.