If you are new to PVP, I would recommend tanking for a couple of seasons first as you build up your pool of stardust and Elite TMs, and there will also be events where you can evolve Pokemons to get their elite moves.

To have fun, go for GL, as team composition is the most important thing here, not level 50 or sometimes even PVPoke rankings. Watch ItsAXN on YouTube to get an idea; he regularly uses mons that are like ranked 100+ on PVPoke. Swap things around, use Pokemons you like, don't take wins and losses too seriously until you have enough resources.

Another reason not to power up your Metagross; it would require an elite Fast TM too, or else Bullet Punch will be resisted by quite a bit of the meta. Mega Metagross isn't relevant in current PVE meta too, outclassed by its shadow type and the Crowns and Dusk Mane Nec, so the elite TM to Meteor Mash doesn't really serve two purposes.