Since you have no back wall, assuming the top of the 13x23 area is your HT I think you would benefit from 7.1 as long as that doesn’t make you compromise on what speakers you will get. In which case I would start with 5.1 and move up to at least 7.1. Reason being is that w/o a back wall to help develop a rear “phantom” soundstage with your surrounds I don’t think the experience will be nearly as enveloping as it could be. While you can always move your 5.1 surround speakers farther back than 90 degrees to each side to assist in creating a rear soundstage w/o the wall behind you, you loose the benefit of the reflected sounds coming off that wall. If you do move the surrounds far enough back then you will likely get a gap in the sound stage between the mains and the surrounds unless you are sitting fairly close to the front. Basically no farther back than the distance between your left and right mains.
The farther back from your mains that you sit including the more rows of seats you have the more you would benefit from adding rear and possibly even wide speakers as you stretch out the soundstage lengthwise.
With 10 foot ceilings height speakers could be nice but I’ve found that if the surround and rear speakers are elevated at least a few feet above ear level then the height speakers don’t add that much IMO.
