Bramble has posted a lot of good info. I would like to doubly recommend the third world squat article. That really helped me.
There are other issues besides just the hip flexors, although I really think they are often the biggest culprits. Ankle mobility and calf tissue quality is HUGE. Even if your hip flexors are mobile and loose, you can still suck if your calves are tight and you can't dorsiflex your ankle a good 15-20 degrees.
Another issue is the ability to internally rotate around the hip joint. Robertson and company wrote an article on it in the past, gave specific stretches to help with all the issues. And I believe gentilcore wrote one as well.
So, in addition to some of the things you are doing now, also lool into ankle mobility and other aspects of hip mobility, not just flexors.