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.
I dunno about that. Not too many people have bothered asking for it.
It is however RAW from TT so it *Should* be the case.
Also, regarding your take on Deepened Pact and Book of secrets - won't that erode the Bard identity with their Magical Secrets mechanic?
Not really. Magical Secrets for Bard lets them fully learn other classes spells, which they can spend their spell slots on and upcast at will. (Also as they get Magical Secrets at level 10 they can get up to 5th level spells)
Warlocks getting things from Tome, gives them 1 cast of those spells each at a fixed level per long rest. Effectively giving them a "Scroll" of the spells per long rest. (Which is mitigated by the fact that Warlocks are normally Short Resters due to their natural ability to regain spell slots on SR)
The only crossover is that both things give spells from other classes lists... But that's literally RAW for those mechanics that's just what they do.
