A few notes:

1) There is no Premium Select on Hawaii flights -- just like other domestic flights. Yes, some planes do have a PS (SLC to OGG in March), but it is marketed as Comfort Plus. So, no need to worry about this.
2) Flights to Hawaii are a combination of Delta One and First Class. Some of the lie-flats (LAX to HNL) are marketed as First Class. This impacts GUC/RUC upgrade codes
3) For First Class Flights, GUC = Last Seat and RUC = Z
4) For Delta One Flights, GUC = Z and RUC = OY (a hidden fare bucket that books into OX).
5) You can find the above codes on expertflyer.com (paid subscription) or by loging into Delta and seeing upgrade availability (sometimes reported to not be accurate)
6) More on this WIKI thread Using Regional Upgrades on Delta (RUC) post 2/1/2022

Hope this helps. Richard