Amethyst darts are easy enough for us to change once they've been live in-game for a while and we can look at the actual data.
Crystal armour upkeep really shouldn't be that bad considering you don't lose charges if you don't take damage. This includes when using the correct Protection Prayers and when you get hit 0. Though again, it's something we can easily monitor and adjust if necessary once we have the data to validate any changes.
I didn't see a need to mention how future Ranged armour with Ranged strength bonuses will interact with the Toxic Blowpipe seeing as it will be as per it is now. The more Ranged strength, the stronger it becomes - but at the same time other Ranged weapons benefit from the same improvements too outside of the Crystal Armour.