Samio over at the modding Discord discovered how to unlock factions. I made a super quick guide over at the tutorials section of the forum along with a sample mod unlocking Welsh factions. Basically you just need to remove/comment out some lines to do with saving and loading in a script file. Saving and loading still works without the lines and the game no longer crashes when playing as a minor faction. Only crash I've had was clicking on the notification/reminder for assigning a provincial governer, you have to just dismiss it and do it manually instead.
One thing to note, I tried also making the viking factions playable but only half of them seem to appear in the menu (along with all the welsh ones I unlocked) but it might be because I made a mistake somewhere. It might also mean that there's a limit in how many you can unlock at once. The box expands as if to show more but not all of them appear.