After 110+ hours of play, dealing with the bugs, the crashing, and needing to redo a 6 hour segment to finish up this achievement, I feel compelled to share some insight. This'll be far from a full guide, and will contain some speculation on my behalf, because I'm not replaying the entire game to figure these things out, and with some bugs I couldn't right now if I want to.This is likely unobtainable on 1.2, UNLESS you finish up Matthew and Fritz's questline BEFORE you head into the Vranik camp curing the main quest. Doing so bugged me out of completing side quest Masquerade, where I couldn't speak with Olena, so I had to revert to a 6 hour earlier save to get this done.
Other quests might be messed up after this point, but I had a good many done before entering Vranik.
See my solution for Judas for more information:
This is probably missable if you fail a quest for any reason. There may be a small room for error, but only if you've done things a certain way, which I will try to explain below, but can't confirm due to bugs.
If we're to believe the list linked below, there is 30 main quests and 56 side quests:https://guides4gamers.com/kingdom-come-deliverance/quests/
I think the trigger is around 70 quests finished for completionist to unlock, as I unlocked it after completing main quest "Out of the Frying Pan".
The first major choice is made between side quests "Hare Hunt" and "Sheep in Wolf's Clothing" where you side with the guards of Huntsman Nicholas Angel or the Poachers. The Poachers are actually the good guys and required to side with them for the Master Huntsman achievement anyways.
Doing so fails "Hare Hunt, but completes "Sheep in Wolf's Clothing". The outcome here as far as I know is you complete one and fail the other. If there is a way to complete both, I think this would give you a +1 toward the required 80 quests complete. But failing "Hare Hunt" is not a detriment as the game expects this outcome.
Likewise during the main quest, I speculate the game may account for a player mucking up the monastery quest line and not doing ANY of the Side Quests within the monastery. So bare minimum, you'll get your +1 for completing the story quests, but you miss out on the (what my guess right now is) optional side quests in the monastery.So if Main Quest = 30
Side Quests = 51
Monastery Side Quests = 5
Completionist achievement unlock = 70
Then my theory is you should be safe to get this achievement without doing a single side quest within the monastery and only the bare minimum to progress the main quest. I can't confirm, and I'm not replaying the game on an alt account to find out.
The other free +1 I think might come during the quests outlined above in my Judas solution. I'm not sure if the game fully intends for you to complete both "Gallows Brothers" and "A Rock and a Hard Place". You can find a solution to get completion for both, but the game will not fault you for failing one or the other. These two quests are WAY too buggy right now to confirm anything, and the only way through them right now is to follow my Judas solution, or to steal the money and split it with Matthew and Fritz. I have no information if "Gallows Brothers" is essential or not, but based on the rough numbers above, I think the game will allow you to fail one without affecting you negatively toward completionist.
I hope this is a great start for you all, This'll take more effort than myself to confirm everything.