Here is the rule of thumb:
Have 8 S before combining for SS (4 allies + 4 helpers)
Have 8 SS before combining for SSS (4 allies + 4 helpers)
If you don't care about labyrinth, then you would do best to not upgrade until you have 6 units as stated above.
There are things that blur the lines such as some S rank MAX+2 having higher stats than SS+MAX. But that is the general guideline.
This game rewards a well rounded team and penalizes a single uber strong character.
If you don't mind me asking what would you do if you were me
Laby - Generally round 20-22
Arena - 2-4% bracket
Mage (current main) -
Helm - Legendary Buster Max - 2/4 good rolls
Gloves Max- SSS - 3/4 good rolls
Robes +5 - SS - 3 decent rolls
Boots Max - SSS - 2 good rolls
Weapon - Buster max - Meh rolls, have a secondary with slightly better
Offhand - Buster Max - 3 good rolls
Accessories - All SSS of various nature
Currently 4/6 Buster. Some of my other SSS are much better rolls than the buster set though.
Mainly going to use her for Dungeon farming / Arena / Labyrinth
Rogue (almost 50)
Helm - Buster Helm - 2 good rolls
Gloves - SSS - 3 good rolls
Armor - Buster - 4 good rolls
Boots - Buster - 3 good rolls
Weapon - Buster - Not sure if good rolls
Offand - SSS - Very good rolls
Accessories - All SSS with solid vigor + AS
Mainly looking to use him for the World boss / Faster SS raid farming
Party
Dark Priest Porin +7 (SSS) - Solid gear all max
Dark Archer Pitch +5 (SSS) - Buster Weapon set Max + SS armor - Max
Commander Arat Max (SS) - Weakest gear of everyone
Dark Lance Max (SS) - Second weakest gear but Max
Subs
Dark Breath - Max (SS) - All gear maxed
Preist Ranran Max (S) - All gear maxed
Sorcere Mer (SSS) - Max leveled him but shortly after got Porin so he has no gear
Desert Gnarl - Max (SS) - Has nothing
Punisher Roq +2 (SS) - has nothing
Was planning to fuse gnarl and roq together for a new SSS. Should I wait?
Looking at this now, I can just put gear on Mer and fuse Dark Breath with Commander Arat or wait for a new SS later on.
What do you think I should focus on?
Sorry for the wall of text!