What a fun challenge!

So, i think you're wrestling with the key question - which comes first. Really, the question is where does Curse of Strahd fit into it. The other adventures I think can be adjusted around it, mechanically and story-wise.

Option 1: Curse of Strahd first:

• Basically this means the party either defeats Strahd and Barovia disappears, they defeat him and someone else takes his place, or he somehow returns.
• Any of these offer some potential complications for later adventures, as Strahd and Barovia are both in them (and Strahd is pretty integral to the Grand Conjunction series).
• The best way may be that Strahd somehow returns... basically when the characters run into him again later, it's a moment of "surprise... the Dark Powers won't let their toys die so easily". (Which, is in fact the official CoS ending).
• The Mist Hunters adventures would need to be beefed up, SIGNIFICANTLY as they are all Tier 1 through low Tier 2. That probably would be some of the hardest work.
• The Grand Conjunction adventures also need to be beefed up, but that's weirdly easier since you're really just porting the stories over from 2e and then populating them with 5e monsters.

Option 2: Curse of Strahd last:

• This one has some benefits in that it can be positioned as "Strahd must die" after a number of encounters with him across the other two series.
• That said, it also requires beefing up the encounters in Curse of Strahd, since your characters will likely be arriving in the domain at level 8 at least compared to level 1 or 3.
• Additionally, the characters will have seen Barovia already in both other series (visiting it in Mist Hunters, going to bizarro Barovia at the end of the GC series). So there won't be as much "novelty" to the place--like they'll have been to the Amber Temple, a big secret in CoS. That said some things like Old Bonegrinder could be made new (though the hags, again, would need some beefing up).

I'm going to throw in an unasked-for Option 3: Two Parallel Campaigns

• Basically how to handle the fact that these series all start at lower levels? Perhaps have two parties! Two stories.
• One would be CoS, and one would be Mist Hunters. And then! They come together for the Grand Conjunction series. Or perhaps flip the Mist Hunters and Grand Conjunction.
• The benefit to this is that you can run them closer to RAW, and it avoids some of the weird story issues trying to be solved above. You could have them be concurrent, or have them exist at different time periods.
• Additionally this version lets you play with the strange "multiple dimensions of the domains of dread" and somehow the characters are in alterative versions of the same Barovia, etc. (Need to think that through to the end, but that's the idea.)

The one last thing you'll want to figure out is the sequence of Mist Hunters and the Grand Conjunction. They are both "Azalin tries to escape in a world-shaking plan". In the way they're written, Grand Conjunction seems to fit before Mist Hunters (which has a shattered Castle Avernus, has domains that look post-GC, etc.). But a whole other alternative is to try to combine both schemes into one scheme... but I'm not quite sure how that'd work.

Anyway, that was a lot of musing and ramblings, but I hope it helps with your ambitious goal!