Compound bow is far superior to the composite bow in the stable branch, and I highly recommend. Carbon fiber arrows are hard to find, and break often enough that it’s hard to use them for an extended period of time.
For the vehicle, yes, go for the ballista. It’s incredibly powerful, and makes the riot control turrets as well as any above ground monster a walk in the park so long as your vehicle is within range.
The survivor armor is better than the ESAPI vest not because it has higher protection values, but because it has far higher coverage. The ESAPI vest has somewhere around 70-90% coverage, but even at 90% there’s a 10% chance for the armor to be ignored completely when you get hit. The survivor suit has 100% coverage.
It depends on what kind of lab it is. If it’s an old science lab (pink map marker) just bringing a keycard to open the door will get you in and from there the turrets are mostly isolated. If it’s a new lab (blue map marker), then i have no idea because I’ve only raided a single one before and it was pretty breezy.
As for additional objectives, setting up and fully completing a survivor camp is a pretty good objective, as is doing all of the quests for both the Hub 0 and the Refugee Center. I recently built a walk in freezer using vehicle coolers, so you could do that as well.