I don't think the availability & "late" evolution are as debilitating as you think it is.Now that I've finished the main game + Episode RR, I'm regrettably nominating my boy Metagross for C-tier.
Availability - This is, admittedly, where Metagross hurts the most. Beldum isn't available until the third island, doesn't fully evolve until around the Aether Foundation, and can be a bit of a nuisance to catch, but on the bright side, its encounter rate isn't bad at all for a pseudo-legend at 20%.
Movepool - Metagross' levelup movepool is good on its own, with access to two crazy strong STABs in Meteor Mash and Zen Headbutt, STAB priority in Bullet Punch, and strong coverage in Hammer Arm when it evolves. Unlike in SM, however, it also gets access to Iron Head, Thunder Punch, and Ice Punch through Move Tutor right off the bat, not to mention Brick Break, Rock Slide/Tomb, Bulldoze, and Psychic through TM for the midgame, and in the postgame, it gets Earthquake. It can even use Corkscrew Crash as of the very area it's found in. Having this many options means you can basically tailor Metagross' moveset to patch up any holes in your team comp, and depending on the moveset you choose to run, you can trust Metagross to hit pretty much anything that's put in front of it hard.
Typing - On its own, Steel is pretty incredible in-game (see: Magnemite in pretty much any game it's in). While Psychic doesn't do it many favors and renders it vulnerable to the many random Dark-type moves thrown around this game (hello, Guzma's Ariados...), it does notably make Metagross resistant to Ultra Necrozma's entire moveset, which is really helpful.
Stats - You don't need me to tell you that Metagross' stats are awesome. It sports a monstrous Attack and Defense stat along with passable Special Attack and Special Defense and a semi-beefy HP stat to boot, thus letting it rip and tear through most things easily and even play with some special coverage options like Sludge Bomb and Shadow Ball if you so choose. Even as a Metang, with Eviolite, it becomes a godlike physical tank, and can eat some special hits, too. However, it is slow, meaning it'll have to take a hit most of the time and may eat through your healing items rather quickly as a result.
Major Battles - ...this is kind of a 50/50. As far as the totems it's available for, Metagross/Eviolite Metang is pretty good overall; the only one it 100% loses to is Totem Mimikyu, which wins 1v1 by virtue of Disguise. Otherwise, it can hit Togedemaru and Kommo-o super-effectively (provided you have the TM for Bulldoze, which you absolutely should) while only taking neutral damage at worst from both, and utterly blows away Totem Ribombee with Corkscrew Crash. As for trainers, it notably rips through Olivia's team and can go 1v1 with most of Kahili's, Lusamine's, and Guzma's mons without much of a problem (along with potentially Giovanni's, if we're counting that). However, it does lose pretty hard to Nanu, Hapu, Acerola, Molayne, and Hau (and doesn't do too well against the RR admins for the most part, either). But what I think sets Metagross apart from most other mons is its ability to singlehandedly invalidate the entire Ultra Necrozma fight, a feat that, to my knowledge, most other available mons struggle to achieve.
TL;DR, although Metagross has some availability issues, evolves super late, and is pretty inconsistent against the game's major battles, its movepool, typing, and stats I feel are good enough to justify catching a Beldum on Mount Hokulani, which is why I nominate it for C.
It's on the third island, true, but there's a fair chunk of game left. First off, obviously, the entire remainder of Island 3 is pretty long including the Skull section (not that that's...very hard...but its there), the Aether follow up, and then the chunk of Poni island and finally victory road. And you'll probably be a metagross by around Aether raid which isn't too outlandish.
Its battle performance is probably more relevant than it, is all I'm putting forth