Permit me to suggest that part of the problem might be the term “microfrontend”, especially the “micro” part. Most apps composing microfrontend examples I’ve seen IRL are decidedly not “micro”. Among other problems, using the term “micro” invites the confusion with components that you address. It seems to imply that an app might be composed of dozens or even hundreds of microapps, just like a backend might be composed of dozens or hundreds of microsevices, whereas in reality a typical composed app might involve two or three or at most a half dozen microapps.

I would suggest something like “portal-based apps” (taking a hint from the proposed <portal> element), or perhaps “composed front-ends”.