Video to come. Although it's been covered elsewhere, thought I'd quickly share what was the limiting factor for me, so as to help other people who may have the same issue as me.
My back is very flexible from contortion work, and I have long had the habit of bending my back as well as my hips to get into (what I thought was) position to begin the press.
I spammed Coach's 'Turbo Charging Basic Hamstring Flexibility' drill of piking down and arching my back to reinforce the habit of folding from the hips, not the back, over the past few days. While stretching it again tonight, I found it remarkably easy to straddle up. Even though I felt like I was planching by keeping my back straight, truth is I twasn't, and up I went several times-- effortlessly.
The moral, as usual, train good habits and reinforce good positions. Otherwise you too may spend literal months exceeding gross strength and flexibility requirements for a given and relatively basic skill and yet be unable to perform it.
I'm very pleased by this somewhat anti-climactic discovery, and looking forward to developing it. Next project, pike press.