As I was saying before, FL (full lotus) sitting posture just seems difficult because modern people live a lifestyle, when they're not being sedentary at a desk job, as walking bipeds that load both of their legs at the same time, gradually stiffening them and not allowing the hips and legs a full range of movement. FL can be as easy as folding your arms across your chest. Study your legs and arms carefully, they're the same!
You just need to develop a habit of allowing your legs and hips a full range of motion. Some of the hip opening yoga stretches are great, and I do them everyday, but I spend a lot more daily time doing kicking drills than static held stretches like yoga . I'll do at least one set of various kicks that lasts 10 minutes, it can be integrated into walking meditation, so there is no excuse not to do it.
There are important details to notice about how I'm doing the kicks.
For a beginner, the most important to notice is keep the circular range of your leg motion in your comfort zone. In the videos, I'm just using a fraction of my range to to demonstrate even small circular sweeps can give a great hip opening motion.
The advantage of doing sets on a single leg with a continuous loop, is it develops your balance, enhances the strength, coordination, balance of your waist movement, leg strength, and the free leg that's doing the circular sweeping, gives it time to get to know the feeling of being fully relaxed, without bearing the body weight load like it's so used to. Your free leg, with daily practice, should start to feel as easy as every human is able to give the same circular range of motion as your arms in shoulder socket.
The advantage of doing the alternating line drills, especially for a beginner, is that before you've developed the balance and strength to stand on one leg for an extended period of time, it won't cramp or strain the weight bearing leg.
What your arms are doing during kicking drills
What you'll see commonly in chinese martial arts version of this kicking drill, is they'll hold the arms extended fully, with elbows locked out, and wrists bent 90deg.
Extended arms are for balance while being on one leg. Being fully extended, stretches lots of arm muscles. BUT, if you continuously hold that stretch, the complementary set of arm muscles get tense and block qi flow.
So you notice how I hold my arms, it's fully relaxed, circular like a bow, very little bend in the wrist. Extended arms out enough to help with balance, but with zero tension in shoulders, elbows, wrists, following proper taiji principles to never have tension. That blocks qi flow.