Similar to general flyout implementation?

Potentially? You'd need your filters to be made with a view.

I considered doing expanding fieldsets on a single overview page, but decided to follow the UI pattern on eBay and Vinted, of using subpages for each filter. I don't think supporting both UI patterns would make sense in a single module.

Roger that. There is a new method for creating facets and view filters - https://project.pages.drupalcode.org/facets/exposed_filters - so I'll look into that and circle back around. Thanks!

Yup, this module works well with Facets made with exposed filters.