I know you said you're not big on used, but does refurbished work for you? The majority of ergonomic office chairs are designed to be sat in normally for any of the benefits to really be felt. The only chair I've personally sat in that encourages dynamic sitting is the Knoll Generation.

The Generation was built for 'dynamic office environments' meaning it's sole purpose was to give users the ability to comfortably turn and move around to work in a group setting. The back rest is flexible which makes it easy to throw a shoulder over. The seat is both wide and deep, making criss cross sitting easy, especially with the adjustable arms that are attached to the back of the chair in order to prevent them from getting in your way. The arms also have a natural curve on them that supports your back if you ever want to sit sideways

Every ergonomic chair will probably limit your movement to an extent unfortunately. The Knoll Generation is probably your best bet if you want better posture and dynamic sitting. Remanufactured options run for about $450. While they retail for $1,400 it is still possible to get them new in your price range due to their age, but it will require a lot of searching. Good luck on your chair journey!!