Stretching can help with flexibility and be enjoyable, it is not needed to improve mobility, however.

https://medium.com/better-humans/stretching-or-resistance-training-which-is-better-for-flexibility-c292107341eb

Also, posture can be influenced by many training styles. I question using the qord 'improve' as there is no ideal posture. Research shows it does not cause pain or injury.

https://betterhumans.pub/stop-telling-people-posture-causes-pain-b96c25bf87dd