Cool idea, but a few counter points.

1. Bastions are supposed to be ruled by the piglin clan so unless he is secretly a pig...

2. How would the transformation into a warden work? Wardens are pretty big and the two aren't related in obvious ways.

What is the knight's unique ability or attribute? (ex. Warden's sound sens, piglin's intelligence & bartering, enderman's look making it angry. What would he drop, what incentive would you have to fight him. (Maybe rare drops, high damage. Maybe a mercenary mechanic where you can buy him for a bunch of gold and he will fight for you?)

Ummmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmmm it is as big as warden and when killed it drops piglins and iron armor + it is a piglin sidekick.

Maybe it could be a "nether warden" in a way where if you crouch walk/crawl, it won't see you, but if you walk/run/jump/break a block/make a loud sound of any kind, it can see you and attack until you crouch walk away again. Maybe it drops gold that it was hoarding and maybe a small chance of netherite scrap.