dnd 5e – Would the spell of a sorcerer's mystical arcanum with lower level spells affect your balance?

I'll admit that I'm not challenged by the 8th-level spells available for a Warlock's Mystic Arcanum feature, especially if you use them for example. I have one or more bards in the party (like me).

(Maddening Darkness seems to have such a large radius that he puts your group out of action, Glibness and Dominate monsters seem to be Bard-covered, Power Word Stun and Demiplane are oppressive, and I'm not sure how I feel about Feeblemind – I saw a claim that the casting is probably less ethical than making your pact at all.)

Honestly, the 7th level spells look much more interesting, flexible and useful. (Etheric, Force Cage, Starry Crown, Plane Shift, and Finger of Death everything look like decisions with much greater flexibility or suitability to occupy niches that can not cover bards, especially the role of the blaster.)

I see from this question that it is not RAW-compliant to choose a seventh-level spell (rather than the eighth level) for their Mystic Arcanum at Warlock Level 15, but that would be the case unbalanced?