TL:DR;
Issues in >>21417 were mostly a result of not having a harness, though not entirely.
Alright. Three rings, two cages, one harness.
No harness:
Regardless of ring size, get the massive hangdown, even worse with the nub. The smaller rings actually stay on and are probably wearable, but it's fundamentally not what I'm looking for.
With a tight harness:
(nub, 40mm, 45mm) Tightening the harness with while the nub is closed on 40mm or 45mm rings causes testicle genocide either instantly or after 1-3 minutes. Absolutely destroying the skin, possibly the actual nut, and also seems to be cutting off bloodflow, though the flagrantness of damage means I didn't keep it around for long enough to really tell. With a less tight harness, both can be attached but they shift until you either get into the no-harness hangdown situation or shift until the harness is tight and it starts to pinch/chafe, probably also hanging out of position.
(small, 45mm) Despite that the extra squashing of the nub having a much thicker section where it's actually inside the ballring (being head rather than shaft) is absent, this is still unwearable. Pressure seems to be specifically on the top of the pon*s more than anywhere else, though it's also pinching the balls.
(nub, 60mm) With a very tight harness, this stays on well. The biggest impediment to long wear is an extrinsic: the gathered balls apparatus gets squashed/driven up in some positions where my thighs are together and I pull my legs into my body.
The harness I have now isn't ideal for this, because getting it to actually be very tight rather than vaguely supportive requires screwing around with single-twists using the 'bottom strap' from behind. I'll try other harnesses.
A cage like pic rel honestly looks super ideal. Having the pon*s section fully terminate before the ball ring would solve most problems and allow the ring to be way tighter, and the ring isn't really doing anything structural for me other than serving as means for the harness to attach to the head more reliably (which seems to be how you'd want it, since the balls are so much less robust than the stick). Unfortunately, outside of cheapnasty shit from china I haven't seen any evidence that they actually exist.