IMO, first of all, I'm not suggesting you to use pistol as SMG backup. Pistols are highly rely on special attacks (you can use them as Sniper Rifle backup), with SMG you only need point blank burst, charge to another enemy, rinse & repeat.
For combat:
You have uber DEX, and I assume (1)you're not going to play ironman (2)you're not going to use throwing knife or Three-Pointer, so 30 point in Throwing (around 60 effective at Lv24) for Grenadier is fine.
Pick Grenadier/Spec Ops later than Lv4/Lv8 is very unwise. With Grenadier at Lv4 and Spec Ops at Lv8 you can make 20s cooldown for flashbang, 10s cooldown for damage dealing 'nades (You need Molly for potential crowd control) at very early game. Also Quick Tinkering at Lv4 have proven its value. Crowd controlling is the first priority, please enhance your damage output later than that. Otherwise you'd find that despite you can take 1~2 enemy out in the first turn, other enemy can take you out in the next turn, remember you have only 3 CON pal.
10 PER is not enough for pistol, and too much for SMG. I don't know you are going to play experimental version or not, that new feat Versatility is a good feat for SMG/melee builds. You can exchange PER for AGI or other stats, invest 50 or 80 in guns for important feat.
I thought nimble is enough for passive defense, when you have trouble dealing crawlers, Uncanny Dodge is also good. You don't need three feat to enhance your passive defense, it's better to max out crowd controlling because you don't evase Electromagnetic Burst or Neural Overload.
For subterfuge & crafting:
50 stealth is good in early game, and a joke in Lv25. Max it out and get bonus from overcoat fabric, balaclava, cloaking device... You have decent crafting score, so make it function. 135 effective lockpicking is a little too much, you can make it 100 to save some point. Quick Tinkering is good for traps, most enemy won't spot a little beartrap in one turn. Uber DEX, decent Mechanics score, synergy with Lockpicking... all good reason to invest 25 Traps for a AP free immbilizing effect.
Consider that energy shields have various addons, 135 effective Electronics is okay, but max out Mechanics & Tailoring don't make sense. Components' quality was capped at 160 max, you need 128 crafting score to deal best components, albeit best components are very rare and you have basement workbench crafting bonus.
As a gunner (to be honest, a bullet spamming triggerhappy) you need 50 Mechanics & 15 Chemistry in early game keeping healthy AMRK (Advanced Mechanical Repair Kit) and bullet supply. Later 20 Chemistry, for handmade W2C bullets.
For Social:
Have 25 effective Merchantile to unlock Blaine's special if you don't want to searching all Underrail for god darned JHP/W2C blueprint...
Hint:
There's a rewarding device provides 20 lockpicking bonus after you complete a special side questline.
P.S. This is my SMG/melee char at Lv20 (with some ideal tweak), I hope you can find something useful for your own build.
-Base-
STR 3
DEX 15
AGI 10
CON 3
PER 6
WIL 3
INT 5
-Skills-
Guns 80 (127 from Versatility)
Throwing 30 (58)
Melee 110 (212)
Dodge, Evasion & Stealth 110 (166)
Lockpicking 45 (101)
Traps 31 (74)
Mechanics 110 (119)
Electronics 37 (40)
Chemistry 15 (20)
Biology 45 (50, I love Focus Stim)
Merchantile 47 (50)
-Feats-
1. Suppresive Fire & Sprint (maybe change it for Hit and Run)
2. Opportunist
4. Quick Tinkering ('nades are expensive in early game)
6. Expose Weakness (very important)
8. Spec Ops
10. Blitz
12. Grenadier
14. Commando (invested 80, effective 93)
16. Versatility (I can receive its bonus when effective Melee beyond 155)
18. Expertise
20. Dirty Kick