You can manually move movies in a collection around and they’ll stay in that order. You have to do it through the web interface.
Grab the little grabber when you hover over the movie and you can drag it around:
It should stay in place after you do that.
For sub collections - those get funny to manage and it’s a popular requested feature: Nested Collections
Edit: You can’t manually sort smart collections though. I do have an “anime” smart collection just based on genre tags and the movies in that Project A-Ko collection also show up there so that’s kinduva way to do sub-collections… in a way. But it gets funny if you want some collections to hide content and some to show content so just be aware of that if movies fall into multiple collections.
