Hi jedibapst, sorry to hear about your troubles!
Your prep sounds good, and your focus on razor technique sounds good. Obviously you had it honed by a profession, so that's all good too. So the strop and the stropping is what comes into question, in my mind.
It could still be too high an angle, not enough lather, etc, but others have covered that.
How much pressure do you use stropping? It should be pretty much none - just the weight of the blade.
Also, where did you get the strop? How was it stored?
I know first hand using an unsuitable strop can wreck your blade, even if your stropping technique is good. That is exactly what I did to my razor the first time I stropped it. When I sent it to be honed, thinking I had rolled the edge, I was sent micro-images of my blade. I hadn't rolled the edge. Instead, it was covered in tiny scratches, like being run over a very rough hone. Something in the leather I was using was eating my edge.
I switched to a proper strop, and haven't had issues since.
If the surface of the strop has been damaged, or embedded with something (which you wouldn't be able to feel - we're talking microscopic here) it will chew up your blade. And if the strop is bowing, then there is uneven pressure on your blade even if you're stropping correctly.
Often times when you buy an antique strop, it needs to be treated in some way and the surfaced buffed before it is usable again.
I would set aside the strop, and try stropping on newspaper. Tape down an ink-heavy sheet of newspaper on a very flat surface (glass is ideal) and try doing 50 strokes. No pressure at all, just the weight of the blade.
See if this does anything to improve your edge. If it does, then it's not your stropping - it's the strop.
And even if it doesn't, there's no way to tell for certain. It could be your stropping, or the edge might be too damaged to be saved without honing.
Buying the Filly was a good call. If you just want to wait until it arrives, try 100 strokes on the leather first. Is that doesn't improve it, try the CroOx side.
If nothing else, send it to be honed. Lynn, or whoever you send it to, will be able to tell exactly why the blade isn't performing as it should.