So, should someone like to try "recycling" their old maps, here a few things I did to get there. With a few pointers that might help avoid unneeded grief.
I put together a reflex_textures_25%_lights.pk3 file with scaled down Reflex textures including newly created light shader place-holder textures. All texture have the original Reflex names and pathing. So that the textures will show and directly work in the Reflex editor after conversion. There is still an issue with scale between GTKradiant and Reflex texture size and orientation, but that may be resolved in the near future, not that fundamental an issue as of now, IMO
Using the Q3ToReflex (r10) converter, player clip is converted and other forms of special shaders ignored (like bot clip, cushion, weapon clip, ...). Alas items (weapons, health, armour, ...) are not yet converted, neither most of the other entities like triggers (JP, TP), targets, even though that is possible in theory. Again, nothing major considering most of the work is in the brushwork, and that is converted very well. "Flimsy" GTKradiant brushes, that bobtoolz/Brush Cleanup might find, actually do not get converted either (had forgotten what a pain creating quirky brushes in Radiant can be, since they keep on breaking, in that respect vertex editing in Reflex is a lot more stable). The trigger brushes for the JPs, I coloured yellow, to let my easier recreate their shape in Reflex, this is necessary since JP and TP trigger "areas" in Reflex are not simply a brush with a trigger texture on it, but actually special entities that just happen to look like brushes.
I should mention that the author of the conversion tool put in quite an effort into automating the conversion of patches to brushes. As one might imagine, this is a highly non-trivial geometric transformation. I only used cylinders in AEpyra back then, they converted without a problem, the brushwork was very intense, though, increasing the size of the .map file from ca. 1.5 MB to 20+MB... so for the first time I used the Brush/Arbitrary sides... function in GTKradiant and created 16-sided cylinders. It may not be "round", but still looks rather nice. Brought down the file size to 1.5 MB after conversion. And these cylinders also look nice in Reflex.
Next screenshot will be from Reflex's built-in editor... and hopefully a working map for Reflex.