I managed to bypass/fix a CTD at the end of the turn by bribing a rebel faction (Sarmatian Rebels) into becoming my puppet state. After gaining vision, I discovered that a city in the north east of the map (Bolghar) was surrounded by other rebel armies raiding the city, which I presume is causing the issue.

I suggest to remove these rebel factions for the time being as they do far more harm than good.