----SOLVED----

I'm running hiveos but it was never an OS issue considering I couldn't even get the OS to run.  I should have stated that.  But It was bios settings and certain GPU installation and boot sequence steps that were causing the issue.  that 1st reddit post above had the answer.  I'm now running 3gpus 1 amd and 2 nvidia and will be getting a 1 to 4 pcie splitter soon to add more.

Here's a paste from the reddit post that worked for me:

Here are the bios settings.

HD audio off

Select onboard devices and select the PCIEX16_3 Control and select x2 option

put gen 2 on the 1. PCIEX16_1 and 1. PCIEX16_2

Serial Port disabled

USB all disabled (usb will work in bios only)

Go to Ai Tweaker and put Ai overclock to Manual

Change your power setting to power on so you can switch back on when power returns

Select Pci subsystem Above 4g enabled ( make sure u have latest bios to see this)

After the above save settings and reboot the system which should boot into whichever os you have.

After a successful boot and your gpu 1 showing you need to shut down add the next gpu and then proceed to boot and check into os again for the gpu 2 being recognised.

After adding each gpu one at a time you will be taken to bios page where you just need to press F1 to enter bios then press esc and continue to boot.

So what I did on mine is as follows after each shutdown.
Boot with GPU 1 in the top most x16 slot

Boot with GPU 2 with the PCIE TO X4 USB ADAPTOR IN THE BOTTOM MOST X16

Boot with GPU 3 with the same PCIE to X4 USB Adaptor as in point 2

Boot with GPU 4 with the same PCIE to X4 USB Adaptor as in point 3

Boot with GPU 5 in top most x1 slot

Boot with GPU 6 in 2nd x1 slot from top

Boot with GPU 7 in bottom most x1 slot

Leave 2nd last x16 slot empty.

So having done the above and adding one card at a time I was able to get all gpu working in hiveos.

wow those are good posts.  looking through those It looks like some people have used the 1 to 4 pcie splitters on these reddit posts.  Maybe a combination of getting more slots to work on the board + this splitter.  I just hope it works without too many complications.  The only setting I haven't tried in bios is setting Ai Overclock Tuner to manual.  Gonna give that a shot and see if i can get more than 1 gpu running while I'm waiting for this splitter. Why is this so hard!!

Thanks!

I bought one of these. 
https://smile.amazon.com/gp/product/B092R64W2V/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1

I hope they help but I'd still love to find out why only 1 pcie slot works on this board.

Hello,

I can not for the life of me get more than 1 gpu running on this MB.  I have an AMD Ryzen 5 3600 6-Core, 12-Thread Unlocked Desktop Processor with Wraith Stealth Cooler.  I'm trying to run 3090 + 1080 gpus but can not get it to work.  Ideally i want to run all 6 pcie slots (5 if i have to) but I can't even get 2 working together. I can only run all of my gpus one at a time. I'm using 1x risers.   I've updated the bios, drivers to the latest etc.  I've set the bios to default optimal settings.  that didn't work.  I've modified the bios countless times setting pcies to gen1, gen2 (not gen3 yet should I?).  I've set the pciex to 1x 4x auto etc... I've put the pcie risers in different various slots to see if that's an issue.  I've set the above 4g decoding on.  nothing works ever! Any ideas?!

Anything else i'm missing?
Thanks!