It would be lovely if Accessories had a command for adding additional slots to a player like Curios does. It's honestly the only thing this mod seems to lack for me at the moment!

Such exists for versions 1.21.6 and up, and is currently undocumented in the docs as a lot of changes occurred within a short time, leading to it being pushed back for the time being. Such will most likely not be backported as its just currently a lot of effort at the moment