Bones of earth is permanent (no concentration). Good for standing on, and walling things off. Maybe use down time to make a maze. Though it competes with mass suggestion.
Mass Suggestion is on the bard spell list, so I don't see much competition. I do see competition from other fun/flavorful spells not on the bard list, though!
Illusory Dragon is solid, paticularly with the weaker level 8 spells.
Indeed. Even though you're likely to take Wish, which you could use to do Illusory Dragon, you could also take Illusory Dragon and be able to create 2 per day with your level 9 & 8 slots! Then again, Prismatic Wall is pretty strong.