7th Gen I need help with team building

I recently started another playthrough of UM, wanting to use different Pokemon than I did on my first go at it. Currently my team consists of:
Charjabug
A-Grimer
Furfrou
Flareon
Brionne


I received a Mareanie in a Wonder Trade with a good nature and all of the egg moves it learns, and I am really considering switching out Brionne for it. Is Mareanie any good, and would it fit within my current team? And any other suggestions for the 6th member of my party?

What Pokemon did you use the first time, so we don't recommend things to you that you've already used

Mareanie/Toxapex makes for a good defensive Pokemon

My last team consisted of:
Incineroar
Totem Raticate (for the lols)
Salazzle
Oricorio-Poms
Azumarill
Crabominable
Currently I'm on Route 5 on Akala Island, my team is level 18-20

Ok.

You can get a Hawlucha very early in the game, and it's pretty good for most of the game and so is worth considering for your last team slot. Something with a priority move that's super-effective against Dragon or Psychic is also worth getting too. Not that I remember specific things that have that in US/UM lol

Hawlucha makes a lot of sense, I was dwelling on a fighting type too. It will balance the team out pretty well.