None of this has gotten me anywhere because I am not a 10x top 1% engineer who loves whiteboard coding sessions and brainteasers and wants to change the world by working 80 hours a week, rather a (slightly above) average one who can get things done in an editor and just wants to earn a living doing what I’m good at.
If that's the frame you're taking into the interview, I can see why they don't think you're a culture fit.
Try this frame:
"I'm a proficient programmer, have leadership qualities, who delivers."
Maybe show them a link to your github with some good code as an example, it will help especially since you say you're weak on whiteboards.
"I can be a bit nervous and don't perform well with whiteboards so I would like to send you a link to some of my existing code demonstrating my skills and ability to perform as a programmer."
And the other comments are right - a month is not long ago, and one interview a week is quite solid. Keep steaming ahead.