I like the idea, me and my friend have been tryin to think of a good system like this and I like the base you have for it so far. If you use a system you're familiar with as a base and try to add these rules in on top and do a few test plays, even if you just have two characters fight each other and role the dice for both of them, it might be easier to pin down a specific number system. (5e is pretty easy to insert new rules into and once you find what works you can just trim out what doesn't.) Maybe use the spell DC formula to figure out dodge, parry, and resist pools. Dex for dodge Str for parry (or dex if using a finesse weapon) and Wis for resist (and CON for health of course). The mitigation stat could be just your bare mod for the corresponding ability score (the one for dodge could be called reflex or agility btw). These numbers may be a little high but it's just a starting point.

You could also have a hit area system where you actually designate what part of the body you're hitting if that would interest you. split it into legs, arms, torso, and head each with their own hp, more vital areas having lower hp, reducing a limb to 0 hp would maim or sever it rendering it useless and someone who sneaks up and slits someones throat wouldn't have to have a bunch of extra sneak damage features and spec into assassin classes to know how to effectively stab them in the head. Armor could also be a major factor in mitigating health damage, giving you the most of your "ruggedness" stat while wearing out over time. Maybe a piece of armor has its own hp, or when it's hit has to make a constitution saving throw against the damage it took as a DC taking an "injury" on a failed save. The armor would have a Condition stat and Max condition effectively acting as health. A brand new Leather gauntlet has a ruggedness of say 4 and a Max Condition of 3, if it through taking damage it fails 3 constitution saves it is considered broken.

That's all I got for now, hope you find it helpful, if I come up with anything else I'll chime in.