Ok so I managed to get it work by doing the following changes in bios:
Above 4G Encoding: Enabled
CSM: Disabled
I also disabled HD onboard audio to free up allocatable memory addresses. Not sure it really helped but I’ve read somewhere that you should disable all the devices, controllers and ports you don’t use.
Please note that first time I’ve changed these bios settings the 7th card did not get recognized but at least HiveOS was able to start. After a manual reboot it just showed up, then on a third reboot it lost again. Again a reboot helped and I’m mining happy now with 7 card.
Not sure how permanent is this because as you can see the 7th card gets recognized on every second boot so far. I may need further tweaks.
Hope it helps some of you!
MOBO: Gigabyte X570 Aorus Pro with 4 port PCIe express splitter in the bottom slot.