Refund dispute with nightmare client advice

I entered into a fixed priced contract with a client. A very small one ($150), something I took because I thought it would be a very simple side project. For reference I am a 3d artists and I had to 3d model a tank (no textures or uvs, just modeling).
I already did another similar work for this exact client, that went without a hitch.

But this 2nd time things started to go astray:
When I submitted the model, he started asking for things we didn't initially agree.
-he asked for much lower polygon count than we initially discussed (less than half). This would be like redoing the whole project. This polygon number is never mentioned in our convo and the previous project was accepted at an even higher polygon count than this one (120k vs 80k)
-he initially specifically asked for the whole passenger back side to be modeled, now he asks to remove it and make a tube going up. This is also like redoing half the work and modifies the initial scope of the project
-he says he never asked me to model the gun on top although the reference image he provided had it and never mentioned otherwise. Also wasted time on my part.
-he says he wants the barrel of the gun to be detachable which I have no issues with as is a simple 2 min fix.

I told him there are some inconsistencies with the initial brief and that I would prefer if the requests were made at the beggining before starting the work rather than after I've already finished working on it, because I need to plan in advance when modeling.

We proceeded to have a discord video meet, where at first, he was talking without pausing trying to stop me from saying anything, and when I finally got to saying something, in a polite and professional manner I might add, he started screaming "Don't fucking talk back to me, do you want to get fired? I'll fucking fire you! You're fucking fired!". He then proceeded to end the contract, block me everywhere, posted a quite dishonest review and asks for a full refund.
I refused the refund, while the only complain the guy still retains while in mediation(as opposed to initially while talking to me) is that the main barrel of the gun is not detachable (a 2 min fix job). All his other complains disappeared, I guess he figured they don't hold. And now he wants a full refund for a job that's 99,99% done.
When I explained this in mediation, he started claiming I breached NDA because I posted an screenshot, of this model I did and that he refuses to pay me for, in my Upwork portfolio, because I wanted it to nicely contrast with what he said in the review(In his review he said it looks like AI and it's bad quality, and it really isn't, not even close. This complaint also disappeared from mediation btw and in our private chats he said the model looks great). I told him I never signed an NDA with him. He continued saying that screenshot is breaching Upwork's NDA and proceeded to say his project is worth 20 million$ and will sue me for damages for 200k for breaching the Upwork NDA. All this circus on a project worth $150. Anyway, I told him go ahead, I don't fall for such intimidation tactics.
BTW, his "$20 million project" is literally 7 assets, all made on Upwork, 2 made by me, at $150 each, while he thinks he can make a AAA FPS game in this manner. There is 0% chance this guy has any budget other than some extra money he's willing to spare, but that's irrelevant

I want to enter into paid arbitration, at this time just out of spite, as I pay more for the arbitration fee than the money the client owns me.
Am i in the wrong for not denying the refund? Am I wasting my time with this or can I win this dispute?