Another solution would be the Skirt Upgrade also upgrading the M20 .50 round to armor piercing ones. Let's say as example that in a vacuum matchup between M20 and 222, the M20 is able to take between 50% and 60% of 222 health before dying. And of course remove the bazooka from the crew.
With even a conservatively timed decrew, it easily does more than that.
On another note, I think my main point is that the m20's durability often doesn't matter. It's most effectively used (after its initial ~2 minute shock window) as infantry harassment on the sidelines (read: against lone capping squads), scouting, and minelaying - none of these jobs really require it to be durable in any sense. Its best defensive qualities are its excellent speed and its smoke, both of which reinforce its utility angle even more. Skirts do allow for slightly more aggressive play and can ease the repair burden slightly, but they're by no means necessary for it to shine in most of its roles. It's called a utility car, and it's one of the few vehicles in the game that I would say actually fulfills that role while not also being a generic "drive around wiping stuff" vehicle, owing to its limited shock window and poor performance against most other vehicles (being at the bottom of the food chain of the actual vehicles out there). It's a utility vehicle, and you're pushed to use it as one, basically.
Of course though, this is just my understanding of the unit. Players will use this unit differently, and if the general opinion seems to be that it needs a change regarding its skirts upgrade then that should speak for itself. That said, I don't quite get that impression right now.