Well, you have no water type. So, that's a good place to start. It may be hard to catch, but it has one of the best priority moves in all of Pokemon when it evolves. That Pokemon is Wimpod. Sure its ability is bad and gets two level up moves until it evolves (Struggle Bug and Sand-Attack). You will have another bug type, but it will be useful for the rest of your journey. With access to poison type moves, rock type moves, and ice type moves, it has a lot of coverage.
This is one of the hardest Pokemon in Alola to find but it is worth it. This Pokemon is of course Mareenie later to Toxapex. It is the way better poison water type in this region and has an amazing move set. Its signature move Baneful Bunker protects the target and poison the opposing Pokemon if protected from a physical attack. Ice beam, Liquidation, and Sludge bomb are all great moves. It can also abuse recover, rest, and light screen.
So, for the fact that you don't have a water type, these two generation 7 Pokemon are my picks for your 6th party slot.