At Wits End About Increasing Flexibility At This Point

I've been trying to work on my flexibility in general, but mostly as a mission to be able to touch my toes like my friends and nearly 70 year old dad. The closest I can get despite everything is right around ankle-ish level before my knee bends. Not sure if it's related, but I also have issues with my heels pulling up during some exercises if I don't do wider stances, namely squats.

I have tried everything I can find to loosen and help the mobility of my hip flexors, hamstrings and ankles. I have done the mobility course pdfs offered by Movement by David multiple times. I did the exercises in a recent video of his with another person that showed incredible results rather quick, but resulted in no change for me. I've been doing exercises by Livinleggings as well lately as a regular part of my exercise routine.

Months to a year, nearly two, and no progress whatsoever with the knees staying straight. It feels like every time I think I finally have the solution based on what I've ruled out by then, I still make absolutely no progress.

Try to touch my toes? Get to the ankle and the knee begins to bend

Try to sit and reach for my toes? Monumental effort to not bend my knees and the closest I get is maybe the ankle, more often bottom of the calves in the attempt

Keep one leg straight and lift the other? Thigh is straight at hip level but the knee begins bending

I am beyond frustrated at this point and don't know what else I can do. I've begun the believe it's just impossible for me. Can any of you offer some advice, tips, exercises, anything to finally help me make progress?