Here's my thoughts:

This is a pretty huge buff statistically, as others have said. However, Tyranitar has similar bulk and sees almost no use running defensive sets at the moment. Let's see what you check (assuming no future buffs):
Non-Steel move Heatran (though if you wanted this buff mainly to add a new Heatran check I'd strongly recommend against, since Flash Cannon/Heavy Slam would just be defaults if this was common)
Dragapult
Tornadus-T
Volcarona
Blacephalon
Volcarona
Victini
Zapdos
Galarian Moltres
This looks like a lot, but on the other hand, Tyranitar checks all this stuff with similar bulk (and trades Diancie's 4x weakness for a slightly worse one) and has barely any usage. I don't think this will be much good for the same reason - no recovery and the need to check lots of stuff leaves it worn down really quickly, it's really passive, and as a rocker it can't pressure the most important Defoggers in Landorus-T and Corviknight at all. You might want to run an offensive set on it, but then it really is just a worse Tyranitar.

Stall would rather Quagsire had more physical defense, since it can be a secondary check to Heatran and beat Tapu Koko at the moment, and 95/85 isn't enough bulk to deal with anything notable, eg:
252 SpA Choice Specs Dragapult Shadow Ball vs. 252 HP / 4 SpD Quagsire: 186-220 (47.2 - 55.8%) -- 78.5% chance to 2HKO after Stealth Rock and Leftovers recovery
252 SpA Tapu Lele Psychic vs. 252 HP / 4 SpD Quagsire in Psychic Terrain: 219-258 (55.5 - 65.4%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
If you're running a specially defensive spread you lose to Garchomp, Weavile and Melmetal, thus rather defeating the point of using Quagsire, so this is a bit of a worthless buff.

I think this is the best option here. +15 SpA is enough to get some notable KOes, as others say above, making Starmie a potent wallbreaker with some nice defensive utility versus threats like Heatran and Urshifu-R. Mystical Fire won't be that useful though with massive four-moveslot syndrome, I'd rather run Magnezone support to deal with Ferrothorn and go Hydro Pump / Psyshock / Thunderbolt / Recover to hit Waters, Blissey and Galarian Slowking hard enough.

Spikes is obviously an improvement. I don't think saying that "sleep users get to do nothing else" is quite true - look at Breloom in past gens. Spikes Tangrowth would just be the defensive version of that. Being a hazard setter with Knock Off is pretty good as Ferrothorn shows. The big thing is that you get major four-moveslot syndrome - you need Focus Blast to be able to check Kartana, Giga Drain to hit Ground-types, and Sleep Powder and Knock Off to not be passive into Flying- and Steel-types. Where does Spikes go? I assume Knock Off personally, since Spikes looks to me like it replaces it as the main free turn abusing move. Also yet another potential setter for hazard stack that, like Ferrothorn and Skarmory, doesn't consistently beat any Defogger. If you really want to make hazard stack more viable, we need a Spiker that actually keeps up hazards against at least two of Landorus-T, Corviknight and Tornadus-T - maybe an Ice or Electric-type would be good?