However, that +1 AC will mean she sucks -5% less at tanking for casters, and a whole lot more in melee parties that can heal.
This isn't quite right. With 15 AC, you'll be hit ~50% of the time. With 16 AC, you'll only be hit ~45% of the time. This means you'll be hit (50-45)/50= 10% less often,
taking 10% less damage. Given that only the last hit point matters in 5e, taking 10% less damage can be pretty impactful as it can result in staying up for a whole extra turn.
This effect is stronger for higher ACs. With 18 AC you'll be hit ~35% of the time, but with 19 AC you'll be hit 30% of the time: (35-30)/35 = 14% less damage. At the limit of only being hit by crits (AC>24), that final point of AC means you take 50% less damage (well, more like 40% because crits deal extra damage).
More on topic...I'm fine with this suggestion, but restricted. I like that companions have builds set in stone that match their backstories. However, Lae'zel's Fighting Style isn't necessarily intrinsically tied to her character; Defensive FS would be equally fine. Archery would be too much of a change though.
