OMG the day I get my middle split, like really really get my middle split I will throw a party. I’ve been working on middle split for years now. Must admit probably not with the same intensity as I worked for my front ones, but maybe not. It’s hard to say without video from years ago.
The stretch you described, are you face down and balancing on your hands or arms? Have you tried lying on your back with your legs open in straddle letting gravity pull them down?
When lying on your back with your legs in straddle it would be advantageous to point your feet towards your head which will give you a really deep hamstring stretch. It would also be good to then point your feet towards the wall you would see in front of you if you “crunched up”. You will feel the stretch in your adductors which is what needs to stretch for middle splits.
When trying for the actual split after I’ve done all the warm up and presplit stretching I always use the pole to hold on to while I slide down as far as I can go into split position (which in middle split is still at least four inches off the ground). This helps keep the form correct and helps to prevent injuries.