Bard with a mind focus multiclass question 5e

I will be making a level 20 character for a D&D 5e one shot that will have alot of roleplay and combat. I want to be a bard plus something else. Either a lore or whisper bard and then either an Aberrant Mind Sorcerer or a Great Old One Warlock. I want to have at least 17 lvls of bard so at a key moment I can use true polymorph and turn into an aboleth. With all that in Mind, pun intended, what is the best combo for dealing damage or Survivability in a fight of 17 lvls of bard (lore or whisper) and 3 lvls of Aberrant Mind Sorcerer or a Great Old One Warlock?

Thank you in Advance.