Halo MCC Game Data Syncing Questions

I’m a long-time Halo player and have been playing since 2008 with Halo 3. I have owned Halo 3, Halo 3: ODST (with Halo 3: Mythic as well), Halo: Reach, Halo 4, Halo: Combat Evolved Anniversary, Halo: Master Chief Collection, and Halo 5: Guardians, all on
discs. Now, I only have Halo 3: ODST, Halo: Reach, Halo: Master Chief Collection, and Halo 5: Guardians.

I took note of how Halo 3: ODST is available as DLC for MCC, and considering I already have the game on disc and that it is backwards compatible for Xbox One, I didn’t have to buy it again just to play it (MCC also says in the Campaigns menu that I have
11/11 missions completed for Halo 3: ODST even though I have never purchased or owned the DLC version of the game). However, in order to unlock the content for completion in MCC, I must buy it again as the DLC. Same deal for Halo: Reach now that it, too, is
DLC for MCC. Yes, I can play the games because I own them on disc, but now, I have all the content from those games available in MCC locked because I simply own the wrong version. I have seen videos of people being able to do parties and game sessions across
multiple games (such as one player in the party in Halo 3: Mythic and another player in the same party in Halo 3, playing on the same game session, despite the two being different games), but I do not see that in regard to MCC. Why is this the case? Can I
not just sync data stored on the Xbox itself into the other games? Is there a service out there that can serve as a similar function to how Ubisoft Club tracks stuff through your Xbox account for cross-game rewards? Because that would make a lot of people,
including myself, quite happy. Everything unlockable in the game, great, keep it locked and have it be unlocked again through gameplay, but please don’t lock it to a specific version of the game. Halo Channel was able to do the cross-game sync I’m asking for,
and about, with how it unlocked the emblems in MCC and the Nightfall armor and helmet in Halo 5 for watching Nightfall. Can't this be done for game content as well? I would love to hear what you guys have to say about all this. Thank you!

:)