Single core performance is acceptable, while multi-core performance is relatively poor due to the low frequency of power distribution mechanisms.
CPU-Z isn't that rigorous of a benchmarking tool, and their "validation" website lumps anything with 36 threads or more all together. I doubt CPU-Z is programmed to properly benchmark processors with so many cores. I would try something more like Cinebench which does a good job at stressing as many cores as you can throw at it.
That said, the 7D12 is a low power variant of normal Rome Epycs, with half the memory channels and much lower TDP and speed. I assume the 7D13 is going to be similarly "low powered" focused with lower clock speeds and half the memory bandwidth. If it's only clocking to 3.2ghz in single thread, I imagine multithread is less than 3.0Ghz for all core. Have you raised the cTDP in the BIOS so it doesn't thermal throttle as fast?
The 7D1x models I don't think are meant to be fast, they just offer a ridiculous amount of cores at low power. I think that's why they're so interesting for the homelab since rarely does anyone need to run all cores at top speed, but we do need as many cores as possible when playing with a lot of VMs.
As for mobo support, that's promising about Gigabyte. Is your MZ32 on the latest BIOS and is it Rev 1.0 or 3.0? There isn't a huge change between the two revisions, but 3.0 adds more PCIe4.0 lanes and is built more specifically for Milan support. Rev1.0 still says it's a 7002 motherboard (with others saying it boots 7003 retail SKUs no problem). Only Rev3.0 says it's specifically a 7003 board. The Tyan S8030GM2NE is specifically a 7003 board with explicit support for Milan and Milan-X. The KRPA-U16 marketing material says "powered by 7002" but claims support for 7003. I bet there's a tiny change in mobo requirements for 7003. Small enough that mobo companies can get around with an updated BIOS but big enough that if they don't specifically program support for non-retail SKUs they won't work. Or maybe the custom SKUs like the 7D13 are more tweaked than the 7D12 causing them to be picky with motherboard.