I’m wondering if you couldn’t figure something out using a lazy-loaded engine. Though from what it sounds like you may want a routeless engine(?) which unfortunately don’t yet support lazy loading. So this content is loaded inside other content on one or more existing pages? A routable engine would need to be “mounted” in the router somewhere meaning it would render into an {{outlet}} so that doesn’t sound like it would work for you, since I think that’s basically the same situation as prember. Though depending on your page content maybe it would be possible to use nested routes cleverly? I’m just spitballing.
Anyway other than embroider, prember, and API requests (whether JSON or old-school server rendered HTML content) engines are the only other thing I can think of.