I think all instruments are deceptively hard… there’s always more to them than meets the eye. Tone is always my difficulty with any instrument, whether that involves bowing (violin), blowing (recorder, pennywhistle), or even picking (guitar/harp). Even when I have the fingering down, I have to work on making it sound nice. Like I said, when I was a kid playing around with recorder, I thought if I had the fingerings correct, then I was playing well. LOL! (I probably would have benefited from recording and listening to myself to figure out that wasn’t the case… as I said in another thread, I think intonation, and perhaps also tone, are something easier heard on a recording than when you’re actually playing.)
I also suffer from bad posture… it is part of the reason I prefer to play standing up for any instrument that permits it– I’m less likely to slouch when standing (partly because it’s impossible for me to slouch as badly standing as I sometimes do sitting– I’d fall over!).
As far as stretching, also don’t neglect your core strength. I found years ago that when I started really getting serious about working out regularly, increased strength in my abs made a noticeable difference in breath control and holding long notes when singing. No doubt the same also applies to playing a wind instrument. (It also helps with holding a good posture…)
I’ve found, as you no doubt have, that good posture is more of a habit than anything else… I have had to get myself used to actually standing up straight, keeping my shoulders back, holding myself up with my core (bonus: abs look flatter when the muscles are actually engaged), and curbing my tendency toward lordosis, etc.
I’ve always been interested in the Alexander Method as it’s supposed to be good for this… never tried it, though.