Roon Rock/core Intel NUC won't power up - What are my options

I’m saying that the boot media is formated differently for legacy and UEFI boot and that you need to reinstall if the hardware requires a change in this regard (old NUC with legacy boot enabled and new replacement NUC with UEFI [and not possible to change to legacy anymore]).

I don’t know what you try to say here. Roon introduced UEFI boot support in late 2022 and not 2019 (back then legacy boot was still the only supported option). RoonOS (Nucleus / ROCK) build 254 production is live!