The issue appears to be resolved for me so I'll state here what I'd done.
I changed from Win10 to Win11, though I doubt that contributed.
It's well known that RTSS and MSI Afterburner conflicted with OBS, however I was under the impression that this was only if overlays were enabled!
Now though, the latest RTSS has the option to use Microsoft Detours to inject.
OBS has also now migrated to using Microsoft Detours as their injection method.
Microsoft Detours is specifically designed to behave nicely with multiple MS Detours injections.
Checking the box to have RTSS use MS Detours, + OBS now using it, has resolved all stuttering issues I was having!
Just that simple change.
I'm legitimately shook at how smooth OBS records now.
I'll also state that this smoothness is while having G-sync enabled, with a framerate limiter set to 60fps (the same as OBS is recording at).
Dropping below that 60fps, while smooth for me cos G-sync, does show a stutter in recordings. It just looks like what dropping below 60fps on a normal V-Sync monitor would look like, so I I find that perfectly acceptable.
tl;dr, if you use RTSS, update to the latest version and check the box to have it inject via Microsoft Detours.