I think part of it depends on where you live too. If you live in California - it will be easier to purchase sheets etc. without having to shop around more for a California King. If you live elsewhere then a King will be much easier. We have a California King and I like it, but would go for the extra width if you could. We got the California King because I hate having my head right up against the headboard and if I don't do that on a Regular King then my feet will hang over the end when stretched out.

Could the dresser work on the other wall facing bed, so bed can be centred, as you mentioned you like symmetry. When art and side lamps are added it will feel even more unbalanced. In regards to art above headboard, below is a example of 3 prints to help with height and spacing. Another idea is to add prints above side tables. Roman blinds would look nice, per first picture

If you wanna accentuate the ceiling slope you could put maybe a triptych but each piece mounted higher to mimic the ceiling. Does that make any sense...not sure how to explain lol. Otherwise, just find a piece you love that’s about 2/3 the width of the bed or slightly bigger...too small always looks bad though

I would suggest bringing in some colorful art, just a few ideas