Alright so I "finished" the game by finding room 46, took me about 10 hours.
Here is an inconclusive list of things I haven't solved/found/unlocked
2nd and 4th permanent slots
about 10 or so rooms
probably tons of extra rooms that you have to unlock
that weird puzzle in one of the rooms where it shows 2 paintings marked "with" and "without"
whatever the fuck the boiler does because I only got it during my last run and locked myself out of solving it due to putting it on the edge of the mansion
the sheet music puzzle solution because I'm missing one of the sheets
anything related to the classrooms assuming they're puzzles to begin with
any of the rooms to use the lever in(though I know which 2 rooms they are)
the diary key
whatever it is that the utility room fuse box puzzle turns on
I found some candles lit in a specific way in the catacombs, I think it's a puzzle?
did not manage to find a single safe solution
didn't manage to light all of the braziers outside
the torch room
I managed to open the red room in the basement during my last run, but forgot to check it out
only found 1 item combination
where to use the deposit key
what do to with the battery
what to do with the dynamite, though I saw a suspicious looking wall in the basement
never did the experiment lab puzzle
never used the boat in the basement
Didn't really think I was even close to finish the game. Was expecting to jump through more hoops considering how many things I hadn't solved yet.
The game is definitely nowhere near Outer Wilds/Obra Dinn and it really could have used extreme RNG mitigation through permanent unlocks or gained knowledge.
Also I never had to use my notes.