For those struggling getting in to this position I have two bits of advice:
1. Use a door frame to support yourself initially and really sit back on your heels and hang on the door. It will burn in your adductors so go easy initially. Once you feel comfortable with holding yourself up with the door frame, start to brace and then let go of the door, even for just a second and catch yourself before falling back, to get a feel for balancing yourself. After a few weeks you won’t need the door frame. 2. Stretch your calves, poor dorsiflexion (closing the angle between your shin and the top of your foot) is common and a pain. Everyone should be able to have their toes about 4 inches from a wall and touch the wall with their knee without lifting their heel. Start at one inch and hold it for 30 seconds on each leg once a day, after a week move to 2 inches. I’ve gone from 2 inches to 6 in about a year and has done wonders for my squat.
Finally, when you see people in this squat position in Asia etc you will notice that their spine is rarely flat if they are fully down in the squat. This is generally fine so long as the spine is not loaded. The aim of the deep ass to grass third world squat is not to hold it with a perfectly straight back, the point is to stretch out the positerior chain and, in the cases of people actively using it day to day, to give a comfortable position to work on things low down without forcing their spine to take the weight and job of supporting their bent over action, the aim is to transition that load into the glutes.
Sources: 2 years of extensive physio therapy to recover for a herniated L5S1 disc caused by a lifelong spinal defect exacerbated by mountain bike crashes and heavy dead lifts. If you have back pain seek the advice of a physio therapist with back knowledge before messing about yourself.