"My fluff. It's up to me.
Just like I fluffed my background (soldier-officer) into Captain of the Avant Guard, the best regiment in the army, focusing on scouting and trail-blazing, with a uniform so smart its got a PhD from the best university in the land!
Yeah, I fluffed it, not the DM. I imagined this whole regiment. The DM is okay with it.
Sure, the DM could refuse. If he were a jerk. "
There are other reasons the gm could object to your fluff as presented and they dont necessitate the gm being a jerk.
As a gm who asks or encourages or allows players to invent whole worlds in my scifi game and who gives advice number one to what's next/how improv GM of "check PC background/backstory"... where we cross ideas in opposite directions is that very attitude that a gm not allowing your "fluff" somehow gets tossed as a jerk.
You just linked werewolves and raging and copy of barbarian mechanics into the game. That cannot be seen as "collaboration" of refusing to go along dumps your other collaborators into the "jerk" category.
Your military bit hasn't got enough meat on its bones to really be accepted or denied imo. I would have quite a few questions either way, before I could assess its impact.
But the WW thing... while I personally like it... the attitude of "its mine, take it or be a jerk" does get you auto-refusal, at my table. At my table, we work to play together, not to issue declarations (and denouncements) of that nature whenever we disagree.
Reasonable people can disagree, with some not getting everything they want without one of them being the victim and the other jerks.
In this medium (Internet, gaming forum, debate) each side tends to exaggerate the other side's transgressions, partly because we are passionate and partly to better illustrate the fault.
In reality, I talk to my DM. He says he's starting a new campaign soon, each of you needs to create an

th level PC. We talk as a party so our PCs don't tread on each others' toes, and we talk to the DM about our 'cool idea!' (TM).
In fact, we probably talk more than the DM wants to listen, because we are invested in our idea while the DM cannot be (because he doesn't know what it is yet) and the DM has to take in four or five such ideas while simultaneously readying the campaign.
This means that the DM is, usually, happy for us to do all or most of the work re: fluff. The DM can certainly pipe up and make suggestions to make it even cooler, tell us or talk to us about how our concept could fit into his world, or even say that a particular bit cannot work in his campaign and work with us to make adjustments. Bear in mind that he's already told us about house rules or campaign specific rules. For example, if the campaign is set in Krynn in the years between the fall of Istar and the heroes of the lance the DM will say that there are no clerics. Fair enough, we won't make any clerics; what about druids? And we get to know what the limits are and then create our PCs in that light.
So, at various pre-campaign start moments, each player will go to the DM and say, "Here's my PC". We can tell the story of our characters, how each mechanical ability makes sense for this PC (I'm super-civilised but I've got anger management issues, that's how my Rage ability makes sense for this PC, what with his werewolf-adjacent heritage).
At this point the DM reviews our characters sheets (for crunch). If I were to say that my rogue has the Rage ability even though it's against the rules because I don't have barbarian levels, he would just say no and I know that so I wouldn't present that. If I were to say that my barbarian Rage works differently than it says in the PHB, just because I say so, then I have overstepped my bounds. If that's something I want then I'd have to explain it to him and
ask, nicely, and take it like a gentleman if he says no.
But if I present him with a character sheet that says 'Barbarian 1', and includes a description of the Rage ability that is copy/pasted from the PHB with no alterations, then he simply has no grounds for complaint! How I fluff my rage for this PC isn't in his purview!
No, I'm not foolish or impolite enough to begin my conversation with, "Here's my PC; it's my way or the highway!", because that would be insane and foolish and I probably wouldn't have a game anymore.
But equally, if the DM finds that my PC is mechanically RAW, he has nothing to object to and I would be stunned if he banned my
fluff, saying that it's his way or the highway! He wouldn't treat me so badly anymore than I would treat him badly.
Even if he said it politely, I would be absolutely stunned and gobsmacked if he refused to allow my fluff even though my crunch was unimpeachable! It wouldn't make sense to me! What would he care? My civilised, anger management-challenged version of Rage in no way messes with his world. My cool idea doesn't impose anything on the rest of his world! There is nothing to object
to.
If my DM was that kind of person, one who inexplicably and irrationally messes with my PC like that, then he probably wouldn't be my DM for long. He wouldn't be reasonable at that point.
What actually happened was that the DM thought it was a cool idea too! Everyone is happy.
