VGC18 building team around M-Mence

I want to build a fantasy core (dragon+metal+fairy) around my favourite pokemons (Salamence, Metagross, Tapu Lele) for VGC18 and upcoming regionals in my country (not US).

I'm thinking Mega salamence with hyper voice/double edge as a mixed attacker. Can metagross fill in the metal type I'm looking for in the core? Maybe I can even get tricky and make it a second mega evolution option and keep my opponent guessing? I have a modest Tapu Lele (my only Tapu with the right nature), I heard it has very good synergy with metagross (Zen headbutt + Psychic Terrain).

If this set up is viable, how should I compliment the core. Is there a good rage powder/spore support? Is amoonguss the only viable option in VGC18? Maybe I need a pokemon to check Mega Kanghaskhan (anything other than Terrakion - kinda hard for me to get that atm).

Not sure how to fill up my last slot - should I choose some powerful bulky water type, or maybe electric type? Maybe I need some pokemon to help control speed, maybe put tailwind on my salamence?
So I'm not sure how well Mence and Metagross synergise too well together. Neither are particularly great without their mega and I feel you'd be better off just going with one or the other. If you want Lele as well, I would personally go with metagross (though mence and Lele also have pretty good synergy), and adding Hydreigon. This would complete your dragon fairy steel core, as well as generally synergise well with Megagross and Lele, dealing with Aegislash, opposing Megagross and (depending on whether you want Tailwind or a coverage move) other steels.

From there you have plenty of options. I wouldn't Kang checks are too important considering metagross has a pretty great matchup against it, particularly in psychic terrain, especially if you choose to add Intimidate. Trick room checks would be a good idea, particularly if you don't want to run Taunt on Lele. Amoonguss is still viable and does quite a few things in one slot (TR check, Rain check, Koko and Fini check and redirection). Lando-T is also (as always) a great option. Tbh it's hard to make suggestions without having more of a framework.
I'm fine without metagross, I spent alot of time on salamence and lele so those two are a must on my team, what do you think will go well with mega salamence and lele?

So I'm thinking something like a fantasy core with some support

Dragon: Mega Salamence
Fairy: Tapu Lele
Steel: ??
Support/redirection: ??
Kanga or TR Check:??
Last slot (what type): ??

What do you think synergise well with physic terrain other metagross?
A steel definitely makes sense here. Which one I feel depends on what other mons you add however. I would say Aegislash, Heatran and Ferrothorn are the best candidates. Aegislash works great as a strong ghost to pressure trick room (which if you chose Taunt as your 3rd move I think should give you enough answers to opposing trick room), as well as in general having godlike stats between the two formes and really good neutral coverage between its STABs and having a great matchup vs Kang. Heatran is better at dealing with opposing steels (obviously very much appreciated for Mence + Lele) and is probably the best Fire type if you particularly value that. It also counters Hp Ground-less Zard Y if your team ends up being weak to that. Ferrothorn on the other hand is a great rain and trick room check, and trades being less weak to Lando-T for being a worse switch in to Ice and Flying moves. They are all fairly functional in and out of trick room, and in the case of Heatran and Aegislash often benefit from Tailwind.

So really it depends on how you'd like the team to function. Cresselia is always a great option, being able to set up/reverse TR, having moveslots available for other great moves like Icy Wind, Helping Hand, Toxic, Swagger, Calm Mind, HP Ground (if the team ends up being weak to Heatran) etc. As for Kang Checks, as I said Aegislash would have a good matchup, though really there are lots of options. I've seen things like Z-Focus Blast Thund-T, Pheromosa (which also loves Psychic Terrain), other ghosts like Marowak. In the past you would also see things like Rocky Helmet Amoonguss to redirect it and get nice chip damage off on it, as well as Rotom-W which could take its attacks and burn it. Other things the team might want would be a bulky water check, something like a strong Electric or Grass, like Koko, Thund-T, Kartana, Bulu, or even something like Gastro for redirecting scalds and pumps. And as for things that appreciate psychic terrain, frail support/glass cannon mons like Pheromosa, Gengar, Kartana, Whimsicott etc. As well as mons that appreciate mon that appreciate other terrains being removed, like Lando to remove grassy terrain in order to spam full power EQs, Amoonguss to remove electric/misty terrain so it can spam spore on grounded mons etc. Also mons that appreciate the sp.def boost from Psychic seed (Zapdos, Drifblim etc.)

I really don't want to make recommendations from here since at that point it would literally be a team that I made rather that I was forced to use Mence + Lele on rather than a team that you made that I helped a bit with.
My advice would be not to worry about frameworks or things like that. Go over your team at the moment, and think of a list of Pokémon this team would have a problem with. Think of some other Pokémon that would fill this gap, and synergise well with what you have. At the end, go over if you have any speed control or other sort of support, but most importantly, playtest. It is impossible to see if a team will do well completely on paper.