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Lots of articles here on dynamic stretching exercises helpful for softening body:
https://lucid24.org/misc/qigor/eight-pieces/e3/index.html (disclosure: this is my site)
The suttas don't say this, but collective experience of meditators over the history of man shows that if your body is stiff, meditation is not going to feel good. When your body is soft and pliable, then energy flows smoothly and it is then possible to access the states of bliss which make long sitting comfortable.
While you're in the process of softening the body to be able to sit, don't hesitate to adopt whatever posture as needed when needed to meditate comfortably. Standing, sitting in a chair, walking, even lying down (but I would use a timer with alarm as backup in case falling asleep).
Sitting posture only becomes important for deeper states of samadhi. At the earlier stages of meditation, any posture is fine, more important to keep body comfortable so the mind can be at ease.
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I would advise sitting up against a wall and otherwise just change posture a lot as you sit.
There isn't anything inherently unwholesome to mindfully changing one's posture.
In general i would use the wall or sit on a chair but i would also sit a lot on the floor to see if i wouldn't get used to it.
It always takes a while to strengthen the supportive musculature necessary for comfortable sitting and the joints can generally be worked even into splits.