The reason why I've done a set list for Pact is because, Larian doesn't seem open to the concept of letting Pact of Tome be open ended.
However, limit the pact feature to those 3 cantrips is way to restrictive, so I went with a middle of the line approach.
Also, regarding your take on Deepened Pact and Book of secrets - won't that erode the Bard identity with their Magical Secrets mechanic?
