I don't think 64GB ECC UDIMM is commercially available yet. I use Geizhals to check for EU availability: https://geizhals.eu/?cat=ramddr3&xf=7500_DDR5~7501_DIMM~7761...

In my experience, it's generally unwise to push the platform you're on to the outermost of its spec'd limits. At work, we bought several 5950X-based Zen3 workstations with 128GB of 3200MT/s ECC UDIMM, and two of these boxes will only ever POST when you manually downclock memory to 3000MT/s. Past a certain point, it's silicon lottery deciding if you can make reality live up to the datasheets' promises.

I am fine with downclocking the RAM; my X870E board (ProArt) should be fine running ECC, I only use 9800X3D to have a single CCD (maybe upgraded later to EPYC 4585PX) and together have RTX 6000 Pro and 2x NVLinked A6000 in PCIe slots, with two M.2 SSDs. Power supply follows the latest specs as well. This build was meant to be a light-weight Threadripper replacement and ECC is a must for my use cases (it's a build for my summer house so that I can do serious work while there).