Grass/steel is naturally well suited to being a special wall, but between it's plethora of resistances and only two weaknesses, its more than capable of checking physical mons as well. Looking at the VR Darkrai, Pult, Kyurem, Gholdengo, Chugg, Iron Moth are all the standout special threats in S-A rank. Grass/steel is best checking pult and gholdengo best while stone walling chugg. Without a dark or ice resist, darkrai and kyurem are targetable but would require more legwork. Moth is only possible with a fire immunity. Ogerpon stands out as a mon grass/steel pretty naturally checks, even if it has low kick coverage (assuming its not too heavy).
With this in mind, three suggestions. The first is Purifying Salt. This does a few things for us- it shores up our matchups into Pult and Gholdengo, both stopping them from blowing past us with boosted shadow balls, stops them from crippling us with either twave or wisp (+ hex), and helps preserve our strength of speed by making sure nothing else can slow us down with twave as well. If we have instant recovery, this means that the only tool Rai and Ghold have to break through us is landing focus blasts, which combined with a potential higher speed, gives us good odds at spamming recover into a miss, even if we switch in on a focus blast initially. Additionally, it makes it easier to aim for checking kyurem due to not being vulnerable to 10% freezes, especially helpful if we are forced into a recover loop due to not resisting ice. PSalt makes it a very capable special wall and works together with high speed to allow for recovery spamming without the risk of status hax.
The second is Grassy Surge. The first and most simple reason for this is just- crazy healing. Rocks resistance makes it easier to run leftovers for that 12.5% every single turn, which again combined with fast instant recovery makes it much easier to switch in on neutral special attacks and shrug them off, or to switch in to focus blasts and fish for a miss. The boosted grass STAB helps utilize speed for a wall, opening the route to offensively checking mons as well as defensively. Finally, the earthquake resistance primarily targets gliscor- between normal and earthquake resists, the potential for chip healing to rival gliscor's own, and boosted grass stab to overcome its healing, it makes gliscor a very achiveable target.
Finally, Levitate. Levitate + a rock resistance makes the mon incredibly hazard resilient, still targets gliscor, and opens great tusk up as an easieir target, due to it often dropping fighting stab.