Is there any way to change the default topic list for a tag similar to how it works for a category? Setting it to Top, for example?
No, this is not possible at the current time. Tags are lightweight containers, use a category if you need a lot of tweaks, permissions, etc.
If you want to change your home page to some tag, then GitHub - pfaffman/discourse-custom-homepage-for-groups: Discourse theme component to change home page based on user's primary group will let you change the home page to an arbitrary page on a per-group basis (you’d need to do something so that people had their primary group set to whatever you want).
Thanks @pfaffman but I use Top as the default sort and wanted tags to behave similarly. Will probably have to do it via a plugin.