I'm not a very experienced OBS user, but I will try to assist. I'm assuming that you are using the same camera resolution setting for the Elgato and ATEM. I have been using the original ATEM Television Studio and am looking at software options as a replacement.
Your Mac is using cpu because it is changing the format of the ATEM input from 1080p 59.94 to whatever your OBS resolution is set. How this transformation is done depends on the software and/or hardware controlling the conversion. It's possible that the Elgato offloads this processing to your video card, thus taking the load off of your cpu. Some capture devices have hardware built in to transform the stream so that their is no impact on cpu resources. There is also a resolution setting for the UVC source in OBS. Is it set to use the original source? If so, OBS is not performing any conversion.
The ATEM is sending the raw video directly to OBS. OBS is likely transforming your video source using the CPU instead of the video card or the Elgatos potentially built-in hardware. Is there a resolution setting for the Blackmagic input? Is there a conversion methodology choice?
Your 2012 MacBook Pro's processor is an antique and likely won't perform well. We were asking our 2014 iMac i5 to grab the ATEM input and stream using Livestream Producer and were limited to low resolution streams. The resources used by the ATEM software and Livestream Producer overwhelmed the processor.
One last hint about ATEM. Our problems were exacerbated whenever we transitioned the input in ATEM. During that process, ATEM temporarily doubles the required cpu resources; it's processing the live and preview programs simultaneously. We could watch our processor go from 50% to near 100% whenever we changed the source, often crashing the Mac and forcing us to stream in lower resolution. The Mac was replaced with a Ryzen 3950x PC, which doesn't even know that it working:) You don't need a Ryzen 3950x. I temporarily brought in a budget Ryzen 2200G PC that is similar speced to the iMacs i5, but in real life had no problems handling what the i5 Mac could not. Modern processors are just way more efficient.