You just literally can’t give it a rest
This is 5e, we must push through 6-8 encounters before we can give it a long rest ...
I chose Captain, because while it always denotes leadership it often means a very low level of leadership (outside of a naval context). Commander is somewhat similar to captain, but seems more strictly a matter of military rank, hence not quite as good as Captain.
Captain certainly denotes rank, though it can be civilian (captain of a ship) rather than military. Either way, that implies legitimate authority. (And how much authority is relative, in a fleet action a captain's following orders every moment, detached he's the highest authority available.)
To me Marshal primarily means either a type of police officer or the highest possible army rank.
Both of which imply a legitimate authority out of place with D&D player characters.
Banneret implies knighthood. Good name for a level 3 subclass
Good or not, it's taken, sure. PDK would've been even better as a PrC.
Warlord is a truly absurd thing to call a low level character, especially one who is not evil aligned since in real life it has an exclusively negative connotation. And yes, it is badass, but it only gets that way by being difficult to attain.
Sorcerer, Warlock, Thief, and Assassin all carry comparably negative connotations, and 'warlord,' since it doesn't imply legitimate authority is actually something anyone with a band of followers and a territory could claim, to the literal definition. But, then, few D&D classes adhere to the literal definition, in the first place.
Really, there is no expectation that any
other class must justify it's name based on literal meaning or connotation.
Herald, Warden, and Sentinel are all cool fantasy sounding things, but none of them particularly have to do with leadership. A herald is, in fact, an implicitly subservient person.
In that sense, Herald wouldn't be all bad as a
support class, (in 4e, "Leader" was code for 'support,' broader than the traditional band-aid cleric, but focused on helping allies, not aggrandizing itself).
Warden has been a class, before, and Sentinel has been a sub-class.