I would go on sounds, Riss or [name_f]Reese[/name_f] sound, and also if you’re happy with [name_m]Emrys[/name_m] getting Em, Riss or Rizz, and [name_m]Rhys[/name_m] probably almost always [name_m]Rhys[/name_m], but could get Rhysie, [name_f]Ree[/name_f], [name_m]Roo[/name_m] etc.
There might be a midway name option - maybe it’s neither?