In case someone sees this old thread searching for solutions on the choppyness on Elgato 4K Mk2 recording on OBS, I found a solution to the microstutters that Thommo still had present after fixing the major desync problem with setting Buffering to Auto Detect.
Go into the Video Capture settings on OBS for your 4K Pro Mk2 and set Resolution/FPS Type to Custom instead of Default. Your video feed will disappear temporarily.
Then set the resolution manually from the dropdown to the output from your PS5 (3840x2160 for me) and then set FPS to 59.94. Video Format to Any, Color Space Default and Color Range Default (you may need to play around with the last three depending on your PS5 output color space settings).
Finally, go to your OBS settings -> Video - Set FPS to 59.94.
I believe the primary cause of the little bit of choppiness every 20 seconds to a min is due to most of us recording videos at 60fps. However, PS5 sends out video at 59.94 fps. By manually setting the capture device AND OBS canvas fps to 59.94, we capture the true output of the PS5 instead of forcing it into 60fps container.