I think Two prevos makes a lot of sense here; if you look at all of the 600 BST mons in existence you'll note that the only legendary mons that are "big" in that group are Necrozma, The Lati Twins, Genesect, Deoxys, Zeraora, Zarude, Heatran and Landorus-I. Now, the Lati Twin and Landorus-I arent even standalone; they have counterparts (which in lando's case are 580 BST for some weirdass reason). Genesect's a mythical with a large amount of lore and history and Necrozma's has a ton of decently unique lore that justifies its decently unique bst. Realistically, Zarude, Volcanion, Deoxys(who has that weird form change gimmick to justify its lower bst; as each form only needs a singular stat to be superhigh and everything else can go to the dumps), Zeraora, and Heatran are the only standalone legendaries that look bulky and naturally have a 600 BST; which is very little when you consider that the majority of 600BST mons are either psuedos or baby mythicals. You'll actually find that most of these "sub-legendaries" or "standalones" are 580 BST.
Furthermore, most legendaries typically have abstract or natural concepts that they're based on; Necrozma and Deoxys have relations to space and cosmos, Heatran is based on the concept of the core of the earth, Landorus is based on Harvest, etc. Grav's lion is based on a very literal juxtaposition of a painting and a lion's mane, and the entire design revolves around the idea of paint rather than art in general; paint is a very literal and human-made thing. Now obviously there are exceptions to this (the glaring ones being Volcanion and Magearna), but its safe to say that the majority of the legendary concepts are either abstract or nature-based.
I also think that Grav's design fits well with the more caricatured and concept-based Pseudos we've had lately; Goodra, Kommo-o, and Dragapult were all huge departures from the pseudo-legendary norm with their more varied personalities and niche concepts. I dont think its out of the question for Grav's design to be introduced as a pseudo, with GF breaking a lot of their design rules with the recent pseudos. Making it a pseudo would also allow us to make the most out of its design in terms of lore, since mythical pokemon tend to have very supernatural backstories that showcase them as the sole protector of a village or a society or a culture; basically a major hero character. Whereas this design feels much more common and grounded if that makes any sense
Finally, this is really subjective, but I think giving it no prevos would be somewhat of a waste of its design. There's so much in this design that a possible prevo could work off of; the snake, the chimera inspiration, the frame, the cubist aesthetic, the gold; Grav put a ton of soul and inspiration into this design and it would be a shame if the full extent we could explore that through was a Pokedex entry.
