I try and sit up with my legs open a lot of the time lol my downstairs area is so painful. Then I'll move to having my feet elevated on something. I move around a lot in the evenings I just get uncomfortable very quickly.
Those are the ‘right’ positions to help but I think it’s not really considered necessary until about 34 weeks - before that I’d say your comfort is paramount!
Whichever way I’m comfy on the day 😂
I alternate between things. I've got a wingback armchair that I put a pillow behind me on that I find the most comfortable. I also sit on my ball or on the sofa. I dunno about any specific positions you shouldn't be in but I feel any way I'm comfortable is preferable!!