Sorry just re-found this thread.
I know all about Gridcoin, and CureCoin, as well as ByteBall (which is actually profitable unlike GRC/Cure), not to mention DCC (SPARC) too.
Damien, from what I have read about this mobo there should no reason that you should not be able to get all the x16/x8 slots working; indeed if you cannot even get a single GPU running on the x8 it shows that the problem is not really to do with the slots/pcie lanes per say.
Because of this I would say the problem will either be:
1) Faulty riser
2) Setting in BIOS to turn off/on x8 slots
3) Not enough power from PSU(s)
4) Faulty mobo
I would also take a read of this post, as it covers this mobo, and some of the comments are related to GPU problems. I had a very quick scan and I think they fixed some of these by updating the BIOS.
https://forums.servethehome.com/ind...ocket-amd-epyc-motherboard.18518/#post-204489
However, first things first, never use a passive PCIE riser with so many GPUs. If you imagine the amount of total watts you are going to pull through the mobo it maybe explains why there are many pictures of burnt out mobos due to this. Always use powered risers; for x8/x16 this can be tricky, but they do exist if you go search for them.
In terms of the PSU, personally we always just connect two or more PSUs and that works fine; it's better to have too much available power than not enough. Also very large watt PSU are stupidly expensive.