Has anyone been able to check if these stack? I have all three Sun Soul pieces, but I don't have Kyosho's ring, so I'm unable to check myself.

If they don't, I'd probably take a 6 cha/1 wis ring (for DCs, UMD and dr breaking) and ditch the neck. Something like Dragonmark/Telvi's + Stalker Ring or Avithoul.