While it is possible the first ISD was built by the end of ROTS (serving as a flagship here or there), but there wouldn't been enough of them to start replacing Venators for a few years. Tarkin getting one early on in the Empire makes sense. Perhaps as part of the promotion after Ghorman. But it would still take a lot of scrapped Venators to rebuild the fleet into ISDs. Especially with the Death Star project still building the station. After they finish the hull and hyperdrives, its mostly just the reactor/superlaser problem for about ten years after they finish the station.
I think it's a fair bet that if there was one operational by the end of the war, it would have been Palpatine's and/or Tarkin's flagship. Since we saw them in an Venator at the end of RotS, Tarkin's ISD from the Bad Batch finale is most likely one of the first to roll off the line, hence my prior conclusion.
On a similar note; the latest we've seen the Venators still in Imperial service is the second episode of TotE, which given Thrawn's rank can't be earlier that 10 years post RotS. They the transition wasn't instant by any means.
As far as resources go: remember that the Empire also had the entire Separatist Alliance's fleet and droid army to melt down and recycle, plus it wasn't so squeamish about planetary scale gouge mining. As ever, the bottleneck for the common materials wasn't sourcing, it was logistics. Why else were they still melting down old Y-wings over a decade and a half after the war?
Anyway; not that it matters, because as I already said, the Stormtrooper armour dates the episodes as post-Bad Batch regardless. How much after, and when exactly in the timeline Bad Batch ended (pre or post epilogue) is still very debatable.
Three-ish years post RotS (
circa CRC 7961) still seems like the safe bet for the former, and ten years (circa CRC 7968) feels about right for the latter since it's when the rebellion started to coalesce, and enough time for Hunter to have aged enough.
