I confess that I am disappointed by the level 8 spells available for a Warlock Mystical Arcana function, especially if, for example, having one or more bards in the party (as I do).
(Maddening Darkness seems to have such a huge radius that it would disable your group, Glibness and Dominate Monster seem to be covered by bards, Power Word Stun and Demiplane look disappointing, and I'm not sure how I feel about Feeblemind. that launching it was possibly less ethical than making your agreement in the first place.)
Honestly, level 7 spells look much more interesting, flexible and useful. (Etherealness, Forcecage, Crown of Stars, Plane Shift and Finger of Death). everyone they seem options with a significantly greater flexibility or suitability to fill niches that bards can not cover, especially the role of the blaster.)
From this question, I see that it is not compatible with RAW to choose a level 7 spell (instead of a level 8 spell) for its Mystical Arcane at witch level 15, but this would be unbalanced?