There already is a cap of 2 mechs per stratagem per mission.

I'm at level 65 and I'd say the whole notion of a single loadout meta died with the OG Railgun. We might have been on the cusp of a Slugger meta when that got nerfed (RIP my beloved, that stagger was so good), but it wasn't as prevalent as the Railgun. I don't see much mech strat usage now, the 10-minute cooldown just feels too long. The mech is less fragile now than it was initially, but the rocket aiming is a lot worse after the most recent changes so it's a real mixed bag. Still potentially worth taking on any mission where you're going to spend a long time on an objective (e.g. civilian extraction, geological survey).

I see a fair amount of variety in loadouts, it's more like there's a range of preferences that end up dictating each player's loadout:

• Backpack choice. Personal Shield is still good even after the nerf (nothing else prevents Hunter slows), but the Autocannon is extremely versatile and effective. These are the two most common backpack choices I see, but there's a fair amount of laser drone too vs. bugs, sometimes Recoilless Rifle backpack.
• Heavy clear tool. Some people bring 500kg, some bring Quasar, some like EATs, or the aforementioned RR. Orbital Rail also appears in this role, though the cooldown is very punishing at higher difficulties. Personally I like the Quasar, but some people really dislike the cooldown between shots and that gets worse on a hot planet.
• Chafe clear tool. The vanilla Eagle strike is my personal preference because it can destroy objectives, but the Eagle cluster strike gets one extra usage and a wider area of effect. Sometimes I'll see Gas Strike or Napalm in this role, but I think the damage over time is currently bugged if you're not the host. Hopefully that bug gets fixed so we've got a wider range of effective options. Don't sleep on turrets either, they can really shine in this role, especially after all the upgrades are in place. It requires that they be placed in a good spot to succeed, but if they are then they can absolutely carry. EMS Mortar, regular Mortar and Autocannon turret seem to be the standouts. On small maps like eradication missions, mass mortars will do a ridiculous amount of work as long as they're placed somewhere that they won't get immediately smashed. Most of the eradication maps have a couple spots that work well for this. Friendly fire can be a concern with non-EMS turrets, but it doesn't take many missions to learn how to pay attention to where the AC turret is placed or listen for the sound of the mortar turret firing.
• Oh-shit button. Orbital Laser seems like the standout by a wide margin. Even with the long cooldown and max 3 shots per mission, it seems to do more work than any other Orbital. I have seen Orbital Walking Barrage used to good effect though, it's the one Orbital artillery strike that seems consistent enough to make it worth taking. 120mm and 380mm still need more help in this regard.

That’s interesting. The mechs feel slow to me when navigating certain areas. Even if you stretch out a mech’s lifespan it means more time being unable to use strategems.

Nope, move fast, don't trigger alarms, and if you do, bring hellfire on them. Mechs are slow and fragile, they do about the same as a gatling turret (which you can also throw to cover your retreat). You can't fight every fight on Helldive, since the timer between drops / breaches is so low, you probably won't get to kill them all before a new one pops in.