[Agda] Reflection with terms in different universes

[Roman]

> Another idea involves lifting all the magmas up to the maximum of their
levels during reflection.

I have a blog post about doing a similar thing:
http://effectfully.blogspot.com/2016/07/cumu.html

Applied to your case it looks like this:
https://gist.github.com/effectfully/9ebd95e43c3a562e233e779269e3f241

I'd start from the placeholders approach, though.