With regards to your issue of getting more toughness for using Squirt’s with Fortress Ballista, you can’t rely on the Captain Crimson variant for toughness without Yang’s Recurve so you should go with a standard GoD6 setup. For comparable toughness, try this build:
I cloned the build in the OP but changed out the setup to use Mantle of Channeling. You lose a bit of toughness and utility but gain Fortress Ballista with Squirt’s, so your damage would be overall higher provided you keep the Squirt’s buff up to at least 6 stacks.
Edit: the set you want is “set 2” in the d3planner link. Also, don’t compare the sheet DPS but rather the damage per hit in the skills section. Also, I didn’t include enough cooldown reduction for 100% Vengeance uptime but it should still be 95% uptime, you can replace an area damage roll or something to get that to 100% uptime if you want.