9 generation old CPU doing computationally demanding real-time video encoding, combine with a couple of color adjusting (CPU intensive filters and effects) as well as Audio filters. And 60 fps... did this EVER work? I wouldn't think it would have on that old/slow of a CPU (for real-time video encoding)
For HoW, you aren't doing any slow-motion, right? assuming you aren't, drop to 30 fps
Turn off color correction on PC until you master real-time full system hardware resource utilization monitoring, and ensuring no bottlenecks. That CPU at 44% in your screen capture is the OBS Studio process only. Other processes could easily be driving system to CPU overload. And systems that old often had HDD vs SSD, which would create Disk I/O bottlenecks.
And there are lots of errors on the BlackMagic video input... not sure if you need to fix that, or if that is a symptom of an overloaded PC
Normally, I'd suggest a NVENC capable GPU to do encoding offload, but on such an old system, not really worth it (unless you can get an appropriate GPU for free/super-cheap). Basically, with such an old PC, you are going to need to master Operating System and OBS Studio optimizations for an under-powered system (beware confident sounding YouTube tutorials on the OS side made by people who have NO idea about what they are suggesting, or do, and fail to disclose caveats, implications, etc)
I tried initially streaming at start of lockdown on a 2 generation newer gaming laptop with a dedicated GPU with encoding offload, and couldn't get it stable (decently optimized OS layer, SSD) ok OBS settings ... Now, I probably could (maybe?), but it wasn't worth it at the time (needed reliable platform .. right now!) so we got a dedicated PC for streaming. That enabled me to focus on presentation rather than technical nuances and fine-tuning