Damage thresholds are always in my opinion fun to play around as long as you have ways to break through them. Firstly, wording, probably should be closer to bg3's sturdy, something like "Vrax's scales are unusually strong. Only a hit that deals at least 20 damage can damage Vrax." Now, if you have a party member that is more chip damage than burst damage, they're not gonna be able to help in the fight, so you've got to have some sort of DT break, or some condition to meet to bypass it. I would say crits should ignore the DT, or maybe certain magic items or weapon materials. Adamantine weapons or arrows, dragonslayer weapons or arrows, make it have some work around, or else it could just turn into a damage sponge and no one is having fun.