Can a high level warlock with Book of Ancient Secrets learn ritual spells higher than 5th level?

Let’s say I’m an 11th level Pact of the Tome Warlock with Book of Ancient Secrets. I’m able to learn spells to cast as rituals from any source, I can transcribe spells up to half my level rounded up, and have just gotten my 6th level Mystic Arcanum.

I do not actually have 6th level slots, but ritual casting doesn’t use spell slots anyway, and I am capable of casting 6th level spells (or at least one). Am I able to transcribe a 6th level ritual spell, or am I capped at learning 5th level rituals?