I don't know if this helps but I always picture myself kicking in a tube.

Legs are a whip. Relax your toes and ankles. A little knee bend is fine as long as it's driven by your quad/hamstring push/pull. I know I'm kicking well when I can feel the water between my toes.

Kicking starts at about 25 seconds in:

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I wrote this, you should read it:
https://www.slowtwitch.com/...n_Swimming_6700.html

Try to wear fins more often when you swim. Those will sort of force your knees to get into the slightly flexed and loose position you need to be in to get the proper whipping motion described above. The problem may be that since you haven't been a lifelong swimmer, you haven't built that flexibility in the hips and ankles that lifelong swimmers have. Fins will help stretch those ligaments in your ankles and hips.

I have more ankle flexibility than anyone needs. I do have weak glutes and tight hip flexors though, which I'm sure don't help.

This is the video I took today in my tiny pool, was just trying to get a look at my kick https://vimeo.com/829112848?share=copy

You're kicking from your knee, not your hips. think about moving your knees 6 inches, ankles 12, toes 18". When you're done kicking your hip flexors should be screaming. Incidentally, your head position is too high, so that puts your whole line out of whack. I like that you're using a snorkel, but you should be looking about 6-12" in front of you, not at the other end. Then put your hands at your sides, and try kicking with your back flush to the surface of the water. If you can't figure out the position, try hooking your toes on the gutter and just float around, playing with head position until you find a neutral line at the surface,.

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I wrote this, you should read it:
https://www.slowtwitch.com/...n_Swimming_6700.html

I normally swim with my head lower, but I've been told it is too low - and I often get water in the top of the snorkel because of it. Can't win hey!

Try thinking about turtling it in - give yourself a double chin to retract it into line. Playing with position while floating on the wall is a great way to feel what's going on.

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I wrote this, you should read it:
https://www.slowtwitch.com/...n_Swimming_6700.html