Flurry
Grants you an ability to launch 3 consecutive attacks with a sword. Each attack reduces the action point cost of further Flurry activations by 4 action points (this stacks 5 times) until the end of next turn.
If any of the attacks misses or deals no damage, the flurry will stop(!) and be put on cooldown for 3 turns and the cost reduction will be removed.
Requirements: Dexterity 6, Melee 30
Flurry is the bread and butter for a sword build, and because of that you want your hit chance (translation: effective melee skill) to be as high as possible, thus 18 DEX or 18 STR. I would consider Survival Instinct to be a bad fit for a sword user:
This is the absolute minimum STAT allocation for a sword user
5 STR to wield swords
6 DEX for Flurry
6 AGI for mobility, you are a melee build after all
5 WIL for Ripper (more on this later)
6 INT for Red Dragon
Personally I would opt for 18 DEX (get veteran feat increase DEX) instead of 18 STR because DEX feats > STR feats, max throwing, get fatal throw AP refund + grenadier + three pointer plus enjoy increased melee crit chance, this would means:
5 STR 18 DEX 5 WIL 6 INT = 2 STAT points remaining, which in your case goes to AGI for uncanny dodge.
Some numbers at level 30, max melee for comparsion:
18 DEX = 350 effective melee, weapon damage = 345% <- got this number from mouse over melee in Underrail.info builder
13 DEX = 282 effective melee, weapon damage = 297%
11 DEX = 255 effective melee, weapon damage = 279%
I don't think an increase crit chance of 30% is worth losing roughly 19.4 ~ 27.1% accuracy (especially with Flurry on the line) and 13.9 ~ 19.1% weapon damage.
Now on to Ripper - for me it's a must have because Fatal throw throwing knives always crit and Ripper will vastly increase its damage. @19 DEX (eel sandwich), wielding red dragon, used focus stim = 31% melee crit chance. Considering the number of sword hits per turn using Flurry I think it's justify to get Ripper for the vastly increased critical damage. On the other hand for some 18 STR (sword: Damage increased by 7% for every point in strength above 6 = 84% damage increase or 91% when using rathound BBQ) sword build Ripper might not be worth it, in that case just keep WIL at 3 freeing up 2 STAT points.
Now if you are playing easy/normal, 11 DEX with max melee is absolutely viable. However with Flurry on the line why take more sword misses just for an increase crit chance of 30%?