I will be making a level 20 character for a one shot that will have a lot 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 levels of bard so at a key moment I can use true polymorph and turn into an Aboleth.
What are the pros and cons of and the major differences between each combination of 17 levels of bard (Lore or Whisper) and 3 levels of either Aberrant Mind sorcerer or a Great Old One warlock?