EDIT: Edited build, according to suggestions and more playtesting.
https://underrail.info/build/?HgMOAwMKAwrCoADCoAAAAADCoDcwHhgkUDYeQAA3bkYMADIBJj_CmhZZAlpLKcKHw6I-woXCtcKkMSvCnsKdwq3DiOKnvgritJQF37w
Ambush works off base points? Does that mean equipment/cloaking devices don't affect the crit? That honestly is a very good reason to take Recklessness earlier.
EDIT: They do. Ambush still not impressive at 6; when your base accuracy is what's lacking; and you're typically flashbanging then killing one by one; flashbanged targets don't have evasion to halve. You also need to shoot at near-melee range to compensate for your lacking base accuracy; missing that Plasma Pistol Aimed Shot equals a F9 press.
Sharpshooter was honestly for the max damage screenshots. I gotta try that once, My only other playthrough (after a bunch of scrapped ones) was with Tranquility Quad-School Psi, I haven't played a weapon build yet. But yes, if optimizing, Sharpshooter is definitely one of the weaker ones. Three Pointer though, yeah, I kinda forgot about GLs. And didn't know Thumper has no STR req. Wow. That definitely is not abusable at all. Still an opportunity cost sometimes; you might need two kinds of pistols; at least I planned to. But yeah. That is very powerful.
EDIT: Yep, Thumper OP. Also, I realized you're not limited to Thumper, since Vitality Powder/Adrenaline + All-In is good for 7 STR Onagers (125% crit dmg >> 175% from crit power); and just Adrenaline can fire 25mm 6-Ammo Launchers. So Sharpshooter is subbed out for Tricky Trajectory. Exact levels depend on when you need the extra AOE dmg; I put it at 20, but you can do 16 or 24, whichever works for you. This lets you avoid needing the Launcher Strap Belt; and keep the Doctor's Belt for the 2AP Psi Booster.
Tranquility is mostly for LTI (for Point Shot) and TK Proxy. That's +10 AP if you use both, +5 if only 1. I don't have that many useful perks especially if I also drop Sharpshooter. I see you took Disassemble and Pack Rathound. I'm never taking Pack Rathound on non-trappers; makes my OCD regarding perfect builds act up. I don't know about Disassemble though; I never lacked money post-Abram missions; and that was without pickpocket.
EDIT: LTI + Point Shot is incredible. but you probably should take one earlier, they're effectively +10-15 AP on average. I think Premeditation ends up being better than Tranquility; if only for the Psi points discount; but Tranquility fits the feat curve better; and ends up being better when you need to use many Psi Spells (+Booster), LTI + TK Proxy + Thermo Destabilization is already +15 AP, if you also use Stasis/Cryostasis/PTD/TK Grounding, the AP increase goes up. While Premeditation is typically just a flat +10/+15 AP, on the respective spells; but that big Psi discount makes it stronger on average.
Escape Artist is very useful; except I don't have Locus of Control and Bullheads are annoying to craft, and can't be used after a CC like Locus can be; so without Locus it wouldn't really reduce my F9 press that much
Most of my F9 presses are when I refuse to cheese with traps and fight fair anyways, Bear Traps need nerf. Maybe if the CC duration also scaled off strength or something; stronger foes are CC-ed less by the traps, and on certain super high strength foes, it only does damage.
EDIT: Three Pointer is still very good; mostly because Grenades are 15 AP and comparable in damage to GLs; so when they also get that 50% crit; its more AP for you; especially with LTI and Grenadier. Don't think I'd find the replacement feats (Disassemble, Pack Rathound, Escape Artist) better. Escape Artist can be good in an Ironman run maybe sometimes; but whatever it does can typically be replicated by careful use of traps and corners, and the F9 key.