Well, you want to have coverage, so I would highly recommend having a variety of moves. Flame Charge and Flare Blitz are both great fire type moves that Solgaleo can learn. For a moveset, I would say to try something like this:
Solgaleo @ Life Orb
Ability: Full Metal Body
EVs: 252 Atk, 252 Spe, 4 HP
Nature: Jolly (+Spe, -SpAtk)
Sunsteel Strike- STAB, signature move, pretty good signature move to.
Zen Headbutt- STAB, flinch chance.
Earthquake- Strong move that can cover things like other Solgaleo, which usually have Flare Blitz/Flame Charge.
Crunch/Flare Blitz/Flame Charge- Lunala will fall to Crunch when Shadow Shield is broken, Flare Blitz can take down other Solgaleo, and Flame Charge boosts your speed, though it doesn't hit as hard.
So please, don't just put only STAB. You need a good amount of coverage to balance a moveset out. If you just had Psychic and Steel moves, what if you go against Bisharp? It is immune to your Psychic moves and it resists you Steel moves. If you have Earthquake though, you could hit Bisharp for SE damage.
Hope this helps!!! :)