Tsk tsk... Art at the least is something which resonates with you, for any reason, great Art is a bit different..... Buy a prepared canvas and some colours you like and have at it. Then hang it up on the wall.... I would suggest a large horizontal canvas close to the width of the sofa for that wall.

I think it's fine without a rug. I think rugs are good when it's a large open area and it helps you define spaces. This dining area is well defined without needing a rug

I think a round rug would look great and make the round deck a feature. The issue is that round is hard to find in an outdoor rug especially in a big enough size. If your only option is a too small round rug then go for the right size rectangle rug. A too small round rug will draw the eye to the the rug and it’s ill fit, rather than the round deck.

A few that you may like, Zahira Carla natural from Rugs Plus, Penta beige from Wild Yarn, Lucia from Miss Amara, Abstract Tribal rug from Miss Amara. It’s really what look you are after that will decide your style. You can see how the rugs look in your room with the Miss Amara photo visualiser.