@withwonderingawe This is going to hurt but in this case pain means improvement.
Do not underestimate the power of wall angels and bird dog. I couldn't get my arms up last year when I first tried wall angels coupled with chin tucks were good for a few months. Chin tucks did leave me in pain, but I've had bad posture for years but it definitely did its job. After mastering wall angels I tried building my glutes because i read that in some cases, strengthening glutes help. Weak glutes were not my problem, it took months to figure that out. Started stretching my hamstrings and hip flexors, (found a yoga routine on youtube) and did some spinal flexibility stretches.
After a month of those I found this video
10 minute daily stretch routine on YouTube .
It's good. Ignore the thumbnail, but it did help me get get better fairly quickly. I should mention throughout the whole process that I was also working on strengthening my core.
Now I'm focused on strengthening my rear deltoids, upper back muscles and stretching my pecs. I still incorporate all the stretches and exercises I did before I figured out that the final piece to achieving excellent posture is those two. After a workout, I definitely stand like I never had posture issues and just being mindful throughout the day to correct my posture, then it'll become second nature. I can never imagine myself going back to always 1 before chin tucks it was a painful few days. Standing taller feels great & breathing feels so much better.
Is my posture perfect now? Nah, but it's getting there. I have been doing this for a little over a year.
Short version: chin tucks, wall angels, linked video, strengthen back muscles & rear delts, stretch hamstrings, hip flexors, & chest.