Not sure what firmware limiters you're talking about? I'm using a cheap ASUS board (B450i-gaming), a Zen 2 CPU and a 6600 XT, and single GPU passthru works just fine for me on Arch using this guide. (I haven't tried VMetal or this new release of Nitrux yet). Yes, some manufactures are iffy about IOMMU support, mostly Intel-CPU and Intel-based boards in my experience, but if you're using AMD you should be fine.
There is something called an ACS override patch, but that's a kernel patch not a GPU firmware patch, and from my understanding, that's for dual-GPU users. Regardless, it doesn't modify your firmware in any way.