A couple of other little technical tricks.

Warm up by playing chromatic scales up to the 12th fret & back down i.e. first finger on 1st fret, one finger per fret, 1st finger on 5th / 9th fret ditto. Do it a couple of times on each string. It'll loosen your whole hand up AND teach you where all the strings are across all the fretboard.

Finger a chord. Lift your hand. Wiggle your fingers to break the shape. Finger the same chord again. Rinse & repeat as many times as you feel like. Do it with different chord shapes. Gets your fingers into the habit of falling into the right places.

With both exercises, the more you practice doing it without looking at the neck, the quicker you'll get the instinctive feel. Looking at the fretboard is like a kid sucking their thumb - instinctive, a hard habit to break. It's harder still when you make mistakes; comforting when you can look back down & get it right instead.

Got something on the opposite wall you can focus on? A picture, or something? Focus on that, give yourself permission to make mistakes, rather than get hung up on them, then go for it & have a giggle when you screw up. You'll probably be astonished by how quickly you improve!

Regarding the voice, looking at your vid, I don't think you'll find much difference between sitting & standing. You're sitting pretty upright. It's only if you're huddling over the guitar that you might run into a problem, because you're closing up both rib-cage & abdomen i.e. lung capacity & breath.

Last piece of technical advice. Which might get shouted down by all the marvellous musicians round here, but what the hell! ;-) Classical music is not the be-all & end-all of anything. But! Classical techniques are usually best-practice for any given instrument, because they've evolved more-or-less systematically over very many years.

I think you're holding the neck very flat. Try playing with a position that has the neck angled through, or just below, your shoulder. It'll take a little bit of getting used to. The point of it is that it will help to maintain your hand in exactly the same relation to the neck wherever you are. With a horizontal neck, you're stretching out, coming back in, constantly changing the angle of your forearm & wrist, and that will play hob with the accuracy of your fingering.

You can play great with lousy technique, but you'll find it easier to play better with better technique. Technique isn't music, but it helps to make it! ;-)