Great question. Content reuse at all granularities is a pillar of CMSs IMO (I’ve done courses on CMS selection, been on selection panels, and written formal evaluations against business requirements in the past). Looking forward to seeing what content blocks bring myself.
I’m not going to do justice to your considered concerns just right now. But I did contemplate this in depth when I ported the Minimaxing theme with demo content a little while back. I used modulars as a hack in a similar way to you, so you might want to take a look for further inspiration. Specifically:
- reusable content components in their own non-routable page folders
- accessing it via the default collection
- modular templates utilising the component frontmatter
I haven’t spend too much time considering Admin UI implications as yet, since none of my clients use it (but they and I both want them to). Empty markdown sections are a little ugly, yes, but that doesn’t keep me awake at night. Conscious design or not, it’s likely a trade-off of some sort.
So yeah, any reusable content that has no obvious relation with specific pages, I put in its own folder. You might also be interested in my thoughts about shoehorning this structure for a content API one day.