alphasquid

Yeah credits do roll after the Quiet stuff as well as the real ending. It's kind of interesting that you're having trouble triggering it as I got to it pretty naturally. Didn't really have to do much to unlock the 45 and 46. You could also just watch mission 46 on Youtube since the story stuff in there is pretty important but that mission isn't of much value gameplay-wise

The only story missions after 43 are 45 and 46. 45 is all Quiet stuff and wraps up her character arc, if you don't care about her then that part is optional. 46 is basically the big crazy real end which you should absolutely do. I don't think you need the Quiet stuff for 46. I think you need to do a bunch of side ops while also building up mother base to a certain level and possibly also listening to certain important cassettes and at some point it should unlock. You'll know when it unlocks when you get contacted to come back to mother base and there is a fairly important story cutscene. Now that I'm writing it out it really is king of silly the lengths you need to go but the real ending is pretty big.

Also get Quiet to max bond level.

You definitely don't have to do any of the replay missions to unlock the "true ending", I just unlocked the last mission and haven't done a single replay. There are just a bunch of weird conditions you need to meet to unlock the missions. Try just doing a bunch of side ops and whatever is marked in yellow and eventually you should start unlocking stuff.