Actually armored sledg is the best one in my experience. With Sprint and good tabis you get enough movement to get around enemies and with super steel armor you can get enough resistance that mechanical damage doesn't bother you. So i would still recommend 6-7 CON sledg build.
I just don't like the concept, or maybe better to paraphrase - I have no fun with it at all. I don't say it is bad or something - it's just no fun for me.
3 CON for a build with a relatively low maneuverability is too savescummy for me.
I have great maneuverability in my siphoner leather armor and tabi boots. And when I'll craft infused one it will get even better.
And I don't like to savescum at all, also I try to use no grenades, or as little as possible.
And you won't hit 20 STR without supersoldier drug which you can get only at the end of the game (and not with your biology skill, i tell you that). With maxed STR what you can expect is to take rat barbecue and inject yourself with adrenaline in combat, then you'll have 17-19 str, meaning only 5-15% accuracy penalty on the hammer.
I can get 100 base in Biology, it is I just don't need it now, or have better options to spent points.
Also, 20 is definitely doable, I don't think Styg would put BH if you can't use it.
Also, if no - I always have usual hammers, not a big loss.
About your build, i think it's good enough to finish the game but you WILL hit a lot of roadblocks where you can get mostly by luck after a lot of reloading.
We will see.
7 PER is almost useless, there just not enough secret places for you to discover with effective 10 PER for it to be even remotely worth it 4 attribute points and 1 feat investment.
Look, the idea for this buld was to rely on stealth and dodge/evasion anyway, so I don't need Con.
Besides, I absolutely don't like to skip sooper sekret parts of the game even if I know all of them, which I don't for Underrail (lol yeah). Builds with Per enough to discover all hidden things is a staple for me, that's why crossbow, or pistol or Psi who have spare points for Per.
I would recommend you to take Pummel and Taste for Blood on your next level ups. Pummel is really good, since under adrenaline you can swing you Balor 3 times for 20 AP each and then still hit someone with Pummel for 10 AP (or taser someone, or take some drug).
I already decided to give Pummel a try, but its usefulness is limited - I'm already using Taser, also drugs cot 10 AP which is a huge penalty compared to doctor's pouch users, so it could be useful if you really need to finish someone, which is rare thing at least on 16 lvl.
Taste for blood also works decently with sedgehammer build since it also gives you one stack for killing someone and with decent sledge build you can expect to kill 1-2 enemies per turn, each one increasing your further damage by 5%, and since sledg deals damage in large spikes, it's a noticeable increase.
I think I'll take defensive or utility feats instead, I think I won't need more damage then I can get with 20 Str and BH or even 18 Str and usual hammer.
So my pick is Nimble, Deflection, and some of these - Super Slam, Pummel, Clothier, Power Management, can only take 3 from 4.