Greetings. This is my second post on this forum, so we are both new to it. :)

Where I work, we have at least a dozen Threadripper builds, some of which are the TRX40 AORUS Pro motherboard. Same processor, and some have the RTX3080's. I will say that the RTX3080 on a number of our boards covered too many PCIe slots since the RTX3080 takes three rear panels of space. As such, we had to go with BMD's 8-SDI input card and a 4-SDI input card - there wasn't physical room for 3 BMD cards. I believe the Quad HDMI Recorder is a PCIe 3.0 x8 card, too. I don't recall the specs on the AORUS Pro motherboard, and I'm sure you already thought of this, but be mindful of what x16 or x8 slots are left open after that gigantic RTX3080 gets installed.

Regarding whether vMix on this hardware can handle all those inputs, I cannot exactly answer that. We use our SDI inputs for odds and ends, and most of our inputs come in via NDI, so network performance was the bottleneck we had to address more than BMD being able to sufficiently handle the in's and out's. The biggest challenge we have is the BMD cards disappearing altogether when we reboot these machines. When that happens, we have to power off the computer, hit the power supply's rocker switch in the back of the computer to FULLY deenergize the computer, then power it back on and then the BMD cards are found again. That's on my to-do list to resolve. It doesn't happen all of the time, so I haven't gotten around to that problem yet.

I hope this helps. If you have further questions, I'll do what I can to assist you.