Thank you both so much! Either option seems like it'll do what I want with a bit of adjusting! It's hard to believe I was overcomplicating this so much, but I suppose it's as they say and all of the world really is just a nail when all you have is a hammer. I guess now I just have to decide between having the triangles that I want or having the list easily expanded using the tags system.
Using details has this quirk that the em2 margin applies to everything, though I've fixed it by removing the * from .indent_details *{
Using Table of Contents gave me numbers that I thankfully already knew how to remove from my past muddlings:
<style>ol {list-style-type: none;}</style>
<<toc-selective-expandable "TableOfContents" "sort[tocpos]">>
tocpos is just a value I made up so I can assign value 1 to Home and 2 to Character Advancement, a bit messy but it seems I can use decimals if I need to jam something in the middle later. This way I get whatever arbitrary sorting I want instead of going purely alphabetical (looks like it uses the alphabet for tie breakers which is nice). Unlike with [[Home|SQPFSRD]] that I can put in details I had to make a Home page to contain {{SQPFSRD}} and add that to my table instead, but most of my pages won't need this treatment (I could just rename the page, but having this option as well is reassuring).
Leaning towards Table of Contents purely for how much less work it is. I suppose now my goal is figuring out how to style the ToC so that it doesn't indent as far, and maybe getting it to use triangles somehow.