You might be interested in SubterminalOptimization's thread on beating the game on DOMINATING with a level 6 character; he found that he could stealth Emporion at level 6 with no trouble at all, so with a good crafted stealth set you'll have no trouble whatsoever even with only partial investment in stealth. Infused Cave Hopper leather armor is a really incredible stealth suit when combined with black cloth and soft padding. At the very least, I'd say you can go maybe half each into Stealth and Traps and do very well.
Get some Biology. The Beast fight is frustrating for a simple melee build without the benefit of gas grenades and bear traps and cave ear or crawler caltrops. Presumably you'll have enough Mechanics, from making your own knives, that you won't need Persuasion to skip the Arke fight, but if you do that fight, it'll be a hassle too unless you come loaded with EMP III.
Even with 10 CON, I still think Conditioning is at best a QoL feat. I'd worry more about not taking big hits rather than mitigating damage in. You're probably better off building as an evasion tank and just using your health pool to mitigate the hits you have to choose to take so you don't take the other hits. I wouldn't build the character as a damage tank, unless you plan to build a quirky 35AP PC and carry a quad-plated tank set to swap into when stealth isn't called for. Knives are very very light, so with Pack Rathound I suppose you'd have the carry capacity to spare.