I imagine that few people actually do this, so correspondingly few experts. When I need an SDI signal out from OBS, I use a Projector to feed a normal output of my GPU (usually HDMI), and then hardware-convert that to SDI.
Also, I tried a BM card with 4x SDI - Decklink Duo 2, if I remember correctly - with the idea of having 4x SDI inputs from cameras and remote computers. Even BM support couldn't get it to work. Several unrelated problems, and then I noticed that the available combinations of settings in their config app wouldn't even allow that anyway. So I returned it and got something else.
Normally, I'd recommend one of these for capture-only, as that's what I got and it still works great! But someone on here had a bad experience on my recommendation - apparently they changed some fundamental parts of their driver and broke it - and so I'm not so enthusiastic anymore. But here it is anyway:
Acasis PCIe capture card supports multiple HD resolutions (such as 4K60Hz), bringing crystal-clear visual effects to the audience. It also supports quad HDMI or SDI simultaneous input, allowing you to flexibly adjust between different camera angles or input devices. Solve your professional video...
www.acasis.com
PCIE CAPTURE CARD AC-4HDMI (Quad HDMI-1080P60).zip AC-4SDI (Quad SDI-1080P60).zip AC-4HDMI PLUS (Quad HDMI-1080P60).zip AC-4SDI PLUS (Quad SDI-1080P60).zip AC-VS2583 (2 HDMI-4K60).zip AC-VS2584 (1HDMI+1SDI-1080P60).zip AC-VS007 (Dual HDMI-1080P60).zip AC-VS014 (2HDMI+2SDI-1080P60).zip AC-VS049...
www.acasis.com