I do many things similarly to you (@cavalierex) and your examples are great. I’m not quite sure what your problems are that you can’t use Scrivener at present?
As well as styles, you should also consider Section Types, as these can be made into templates and work in a slightly different way to Styles. One advantage is you can use metadata to define settings for these sections (in my quarto sample, Quarto multi-part figures):
Scrivener can work well with Quarto already, at least for my sample project most things work fine. The ScrivQ template also can deal with multiple file outputs to work with book/web page outputs: ScrivQ | A template to control Quarto, export multiple files, manage bibliography and easily create cross-references
Can you give a more specific problem[s] that made you end up using BBEdit?
I think Pandoc is well-established and universal enough (and now <20MB compressed) that it should be bundled in Scrivener 4 to replace MMD as the default markdown engine (users could manually install MMD easily for backwards compatibility). The numerous feature advantages are significant (citation processing alone would be worth it). While I do really like Quarto, its syntax is much more specialised, targeted at a very specific layout style. If Scrivener 4 supported it it would be awesome from my perspective, but I can imagine it would be much harder than make co-exist with other output formats. In addition one really annoying “feature” of quarto is how it is designed for a single project folder. It refuses to use filters or extensions or other files outside the destination folder by design. Scrivener normally takes control of the destination folder and thus there will be plenty of scope for file collisions and lots of bugs.