I agree with most you said, I only saw it as a possibility since Oda never debunked it, but it's more reasonable to stand on the side that it's unlikely to happen
It does make sense cuz Red-Hair Pirates are probably really good Combatants & Haki Users, while other Yonko & WG also have many DF Users, so it would be weird if Blackbeard didn't make sure his Crew wouldn't be countered by other DF Users,
Blackbeard planned where to find Strong Crew Members, and then He planned to help them find suitable DFs, and nothing says that's the end of his ambition for them, He probably have another Plan to turn them truly unstoppable,
However, I don't think it's Haki Path (Even though They are probably great CoO/CoA Users), I believe Oda already teased their Upgrade, when He made Blackbeard encounter Seraphim, that scene imo cannot be to hype Seraphim, especially when that Arc is gonna show SHs facing them anyway, I believe Blackbeard decided that it would be amazing if He can turn his Crew into Pacifista/Seraphim, hence why He sent Devon to Egghead to initiate his Plan,
BB Pirates with Lunarian/Pacifista Upgrades make more sense than Warlords with such Upgrades, cuz it's not like They are deceased, we already have Mihawk & Mr. 1 still Active & Important, we still have Crocodile & Moria & Jinbe & Hancock, so imo Warlord Seraphim were only created to pave the way for BB Pirates Upgrade.