Ho ok, well yeah, the extreme military power of merchant families comes from their huge potential for retinues.
That got nerfed with old gods. I remember having like a 5k retinue and then old gods snuffed me down to 2k.
Made changes to retinues to be slightly more balanced for republics
Honestly, Sword of Islam can be ignored if you don't want to play muslims. Sunset Invasion is USELESS except for alternate history exporting to EU4. Legacy of Rome changes Military up, a lot, by adding in retinues.(Most direct change to the game there)
The Republic is nice, but if you play a MP game, Republics can bankroll their buddies. They make shit tons of money, if played right.
Old Gods is fun, but I feel they added way to much BS in it for the 867 start date. Allows conquerers to rise up with their own armies to slaughter you, stuff like that. Recommended.
Honstly, Old Gods, Republics, and Rome are your best 3 DLCs. Of those, Old Gods and Rome if you want only a few.