Sounds like a Mac Mini M1 would be a good upgrade in your case. You need to get the 16GB because it has a unified RAM meaning there is no extra RAM for video.
Remember that you’ll need an external PCIe enclosure for the DeckLink Duo 2.
I just receive the mac mini, with 16gb
After a creepy decklink duo 2 firmware update due new BM video desktop, all seems be working ok with ML 6b5
I did a little test 720p59.94 capturing 2 sources, switching, recording, streaming and sdi playout and no dropped frames.
powerfull little beast