Actually, there were already turmoils between Eldian clans, iirc. I bet 7 clans rebelled against the Fritz family, thus joining Marley's side. One of those was the family bestowed with the armored titan, the Braun family (that's why Rod's serum was entitled "Armored - Braun".
If you guys think about it, the "honorary Marleyan" is a status created when Zeke was already born, so which Eldians were in possession of the 7 titans? Clearly, they weren't warriors, as they were chosen merely 30 years ago.
Hence, I guess there were Eldian families who allied with Marley 110 years ago, and they became special eldians to Marley. Yet, I don't understand why those families haven't ever helped their compatriots up until now