I have around 200 engrams waiting to be deciphered at the helm. I try to knock it down 25 at a time but it just keeps growing. I know orbital prison legend is a rough mission, but they'll never allow that.

Just put a dump chest next to the vendor, like the one next to Xur in Eternity. It's not like the rewards are any better than what you get from Dares runs.

I believe at the very least, they should allow us to consume all our keys at once, either by holding X one at a time per key, or letting us hold another button like Y on XB or Triangle on PS for 3 times as long as normal, to make sure you really wanna spend all your keys at once. Maybe even have a new seasonal upgrade at the vendor to go alongside such an upgrade, where spending all your keys at once when at max stacks, doubles the chances of getting red-bordered weapons per key spent. But if you already have all the weapon patterns, it instead gives a small chance to double all loot drops earned from each key, so each key has a chance to count as two keys in this example. As for increasing the key cap, I don't see much reason in extending much higher than it already is. Maybe bump it up from five to seven, in keeping true to Bungie's favorite number, and ten at the absolute highest for sure.