My main business is live video.
My cMP is one of my video switch (the other one is a broadcast grade Blackmagic ATEM)
In my very first cMP version (5.1 single X5679, 32gb ram, GTX980, High Sierra), it was my prime and only video switch, it had:
1 BM decklink DUO (2 SDI channels)
1 BM decklink DUO 2 (4 SDI channels)
1 BM decklink mini recorder (1 SDI or HDMI channel)
The mac was capable to live switching between this 7 inputs, plus 3 monitor outputs, plus PRORES 442 recording, plus youtube streaming, all at the same time flawlessly.
My currently cMP (see my signature), is my small jobs video switch and editing machine, and it have:
1 BM decklink DUO 2 (4 SDI channels)
1 Inatek KT4006 USB 3.0 PCI card with 2 very cheap Chinese USB capture cards
Support maximum input resolution up to 4K;. HDMI Resolution: Max input can support 3840×2160@30Hz / 4K. But other software may not support it. Support maximum output resolution up to 1080P;. HDMI to USB 2.0 Video Capture Card.
www.ebay.com
The mac is capable to live switching between this 6 inputs, 2 monitor outputs, plus PRORES 442 recording, plus youtube and facebook streaming, plus output to zoom via virtual camera, all at the same time flawlessly.
I know this is not the best usb capture device in the market.
It have some color difference problems, but for computers capture, works very good OBB.
I would recommend you BM decklink mini recorder (1 SDI or HDMI channel), but you have to know the device have a strong DHCP protection (can be bypassed with a cheap HDMI splitter).
A single capture input and recording is absolutely no problem at all for a cMP.
Maybe
@fuchsdh don't have powerfull enough GPU