Some more notes...
From a simple listen (no FR sweeps), the Stage 5 is appreciably better in terms of extension to my ears. The soundstage feels wider to me. It has the same kind of forward projection of vocals as the Stage 3, but is quite a bit more neutral-sounding across the hearing-sensitive 600-5k mid-band region.
BTW, the Rhines Stage 3 might sound different from whatever you heard, because I noticed it uses new generation woofers (looks the same as a CI, but is a different family and model number)
The Stage 4 is pretty warm to my ears. Of the CIEMs and CIEM demos that I've heard, the Stage 4 sounds most to me like the UE18, except with a slightly less delicate treble sparkle, but less scooped mids. Similar mid-bass signature. The Stage 4U, has more sub-bass emphasis and more midrange emphasis, including better mid-high presence. The Stage 5 sounds more even; more of a Stage 2 type of sound, but with a better, more three dimensional soundstage and better forward projection.
To me, the Stage 4U sounds more similar to the Stage 4, and that makes sense, considering they probably use a similar driver setup. However, the 4U can feel slightly dry, whereas the 5 feels a bit more alive.
I would say it's less U-shaped than the 1Plus2 --- it doesn't quite have the visceral bass feeling of the 1Plus2, but is more refined and less peaky in the treble.