He has used gear 2nd and gear 3rd against Moria

I don’t think there’s gonna be a G6, Luffy already said his awakening is the limit of his powers.

Yes he can

Snakeman is literally him using gear 2 and gear 4 at the same time

he also used gear 2 and 3 at the same time on multiple occasions

Personally I feel like g5 is his peak but we never know with Oda. After all he did disguise luffy’s true devil fruit identity for like 1000+ chapters lol.