7th Gen Pokemon Moon final team??

Hi, so I have Pokemon Moon and I'm currently at the mountain before the Elite Four, just got there. My dilemma is that I planned out a final team and just realized that it's not going to work out, and I need help with my 6th team member.
My team right now is: Decidueye, Salazzle, Slowbro, Garchomp, Vikavolt, and Magnetzone.
I planned on having Weaville replace Magnetzone to balance it out, but I just realized that getting a Razor Claw is not plausible anymore (I didn't catch any of the dragons in the trial). As much as I love Magnetzone, I used him last game and I already chose Vikavolt, and I don't want two Electric types.
For my new 6th team member I'm considering the following, but have my concerns:
Sandslash (I like him but there aren't any good physical ice type moves)
Umbreon (I already have a Lv. 40 one that I used until I got Gabite)
Absol (type coverage)
Froslass (already have a ghost type)

I'm just wondering what would be the best option to balance out type coverages, and what movesets would work well for the Elite Four. And also, for Slowbro, what water move can I get that replaces Water Pulse, and what ground-type move can I get to replace Dig in my Garchomp?

I only used 5 Pokemon in the Elite Four because my 6th Pokemon fainted so fast. Out of the Pokemon you listed, I would pick Absol against Acerola who uses ghost type. I know Decidueye can use grass, flying, dark, and ghost type moves but you should also let him rest a bit in the Elite four and use him for when you go up against a stronger opponent.

Scald and Surf are good water type moves for Slowbro but he doesn't learn them from leveling and you would have to find the TMs. Sadly, the strongest water type move Slowbro can learn is Water Pulse. I used Golduck in the Elite Four and I gave him the TM Scald since he also doesn't learn that move from leveling. The only other ground type move that is stronger than Dig Garchomp can use is Earthquake and again, you would need to get the TM to teach Garchomp Earthquake. I didn't use a ground type Pokemon much in the Elite Four so you don't need to worry about Garchomp losing PP fast.