Thank you, Mads,
Apologies for my late reply.
I used OBS’s remux feature (located under File -> Remux recordings), which worked well. Managed to FF at x4 with no hiccups (but I noticed some at 8x, but hey, with the file I’ve got, I’m not complaining!), which I was worried about with the .mp4, being a compressed format.
And I just tried your advice regarding version 19, sadly, it didn’t work either.
Anyways, cheers!