There are no mobs in the game with that much health, so what's the point? If you can't move to their position then the damage will be not 2-4k, but 0 ^_^
Health is usually not the issue, mechanical damage resistance is. There are critters and heavily armored guys near mechanical damage immunity. Unlike somthing like a xbow user, a sledgehammer build is actually very good in overcoming it because it ignores damage threshold, damage is done in large spikes, bonus electrical damage is really good and Expose Weakness turns everybody into a paper bag. I'd still say you can never have too much damage.
Mobilty can indeed become a problem. There are many open areas with little to no opportunities to kite behind a corner or a doorway. Stealth pays off big in Underrail, even for a 50% armor penalty guy. Anything above 60% is not worth it for melee IMO. Heavyweight critical bonus is of little use if you need most, if not all of your AP to reach the target. One exception that comes into mind is the arena boss.
For medium/high armor penalty builds it's actually better to use striders instead of tabbis (beware of sneak penalty). Movement speed increase scales with spring quality, eventually reaching 30+ % in speed increase. That's alot better than 10% tabbis have. Sledgehammer also doesn't really need to rely on tabbis -1 AP bonus. The only thing that becomes unusable is hitting 3 times/turn with a tichrome hammer because of 17 AP per hit. Still not a big deal as you can use the supperior tungsten two times + Pummel and for the turns where you don't have Pummel you use a tichrome serrated knife as your secondary weapon. With as little as 6 DEX (you need it for Cheap Shots anyway) you hit with it for exactly 10 AP - perfect alignment with a 20 AP tungsten hit and same AP like consumables. However I have a few problems with a knife in this build: Weak synergy with STR and they don't apply for neither Bone Braker and Heavyweight. A leather combat glove is naturally much stronger here and in the case of the two unique leather gloves if offers great debuffs.
The problem with the gloves on the other hand is, at 7 DEX without tabbis they hit for 11 AP so doesn't fit with a 20 AP tungsten hit. Luckily food offers flexibility here. The solution is to keep both, good striders and tabbis on the character and mix accordingly with either eel sandwitch or rathound barbeque.
When you need mobility: Eel sandwich -> 8 DEX + striders -> 10 AP hit for gloves.
When you need damage: Rathound barbeque -> 16 STR + tabbis -> still 10 AP for gloves.
At this point in buildcrafting it might be a good investment to sacrifice Pummel for Wrestling. When you fight vs. dangerous 1-2 enemies you can't kill quickly it's usually better to put strong bufffs/debuffs first - Taste for Blood, Vile Weaponry, Bone Breaker, contaminated (the Claw) and Wrestling which is good vs both melee and ranged. Cth debuff is flat - it can't be compensated with high skill and I don't think that anything is immune to the tackle debuff. All other Feats are still more important so I'd take this as the very last one. If you'd prefer Pummel instead of Wrestling take it at level 2 and Opportunist at level 22. As a heavy trap user I'm personally sticking to early Opportunist.
It's too bad that Juggernaut doesn't count total armor penalty but only the suit. Otherwise this build would be best played with an infused rathound leather armor with 50% armor penalty (would be possible with metal helmet). Top quality can reach up to 15% critical chance which would in this case double the chance! Good critical damage is only as good as the critical chance it requires.
It can be done by going low on armor penalty and dumping Juggernaut along with Heavyweight, favoring DEX instead of CON for even better critical chance but then it doesn't make sense to focus on sledgehammer anymore as there is much less hitpoints without metal armor + sturdy vest. It would suit better a metal glove/power fist build.