I'm gonna attempt my first serious playthrough of Insurgence once the final update comes out too!
So you got:
Sylveon/Espeon
Mega Gengar
Gigalith
I think a physical ground type like Gliscor, Swampert, or Garchomp would have great synergy with your current team since many physical rock resist mons are weak to Earthquake. The latter two won't add another water weakness as well. Also having something that can switch into thunder wave is convenient too.
Next, a fire type would really round out your team. I'm gonna be a bit biased here and recommend one of my top 3 favorite mons of all time, Volcarona. Volc gives your squad a solid setup sweeper who can 6-0 many teams after a quiver dance. Also, a fire type in general can really help put a dent into steel types like Skarm, Ferrothorn, or Scizor who might give the rest of your team a bit of trouble. If you're not a fan of Volc, you could also try Typhlosion, Arcanine, Mega Charizard X/Y, etc...
Last but not least, a priority sweeper with swords dance seems like a good idea. There are a lot of options like Bisharp, Weavile, etc... but Scizor seems ideal since it also has recovery in the form of roost. However, any decent priority sweeper will do so feel free to pick which one looks coolest
Edit: I just remembered this is Gen 6 so Talonflame's Gale Wings still works regardless of HP. Knowing this, Talonflame can fill both of the last two roles I mentioned in one mon if you teach it swords dance and roost. However, this depends on if Suzerian releases that item from Zeta/Omicron that can give any mon its hidden ability in the next update. If you go with Talonflame, your team is pretty much complete in 5 mons so maybe just a water resist is left. Milotic are Hydreigon are some of my personal favorites that can switch into water moves well, but again feel free to pick any of your personal favorites out of water, dragon, or grass.
Good luck, I hope things get better soon for you ICSW! As someone going into my 4th year in university, I can relate in regards to mental exhaustion/lack of energy. btw I recently started this game and I really like Spork. The pokemon selection is really fun and various, which is a nice break from the limited selection of Reborn/Rejuv/Desolation.