Reach, CE, 2, Campaigns refuse to work

So I’ve been reading the books and wanted to play through in order. Bought and downloaded MCC back when it was first announced, played a couple dozen hours of multiplayer and haven’t really touched it since. Tried to boot it up today to fill in the gaps it takes from the books and…

Reach wouldn’t work. Resume from mission 5 where I was? black screens to load then just says “leaving…” in the bottom right. and returns to main menu. Mission select the first mission? Same issue. Quickstart the campaign? Same issue.
Edit: Also attempted reach multiplayer. Queues into lobby, just to immediately force leave upon matchmaking once the lobby is full.
So I try CE figuring I’ll fix it later and it show the loading screen, cuts to black with music playing, and visual is never restored, can’t tab out, etc.
Halo 2 then? UE4 Fatal error.
Didn’t bother trying the rest

Attempted so far:
Validate steam files (867 failed and were redownloaded… didn’t fix anything lol)
Validate again? (all good this time, still didn’t work)
Delete Maps and validate to reinstall them maybe? (Still no change)
With/without Anti-Cheat? (Zero difference)
Edit: Reach fully reinstalled. (no change)
Edit: Disabled internet. Launched steam offline. Launched MCC without anti-cheat. Attempted quickstart. (No longer leaves immediately but blackscreens for a minute or two before closing the game)

Playing on steam, Windows 10, R9 3900x, RX 480
Can’t seem to find similar issues anywhere else (aside from the issue with Halo 2), does anyone have a clue whats going on here?