This thread has a lot of info on AM5 & ECC:
In general, all AM5 CPUs (so far?) support ECC, the issue is with the motherboard, which has to support it too.
DDR5 comes in two variants RDIMM (registered) & UDIMM (unregistered). AM5 only supports UDIMMs. They are physically incompatible (different pins). UDIMMS come in both ECC and non-ECC. RDIMMS always have ECC AFAIK.
I’m not sure if ECC is working on Asrock (except for the Asrock rack server boards). But there are some members here who confirmed working ECC on Asus boards.
Sata does not seem to be a priority on most AM5 boards, but I don’t see the point in spending a lot to get more sata since there are m.2 → multiple sata adapters or pcie sata controllers that are a cheap upgrade in a $200 board (compared to getting a $500 one with only 2 or 4 sata ports more).
I am a bit puzzled why you would need such a beefy machine for direct 4k playback? You would pretty much always be limited by network/internet, unless you have some crazy 10Gb+ connection and serving 100+ UHD bluray quality streams. And even then you would be limited by storage rather than CPU/memory?