For example, I don’t know if I’m better off using three buff slots on my illusionist to buff other characters (haste, extra mental damage on cast, etc) or the illusionist pet.
Illy pet is for solo only. You are vastly better off using that mass of conc slots on buffing the group. There is no possible way for the Illy pet to match the DPS gain you get from buffing your bard and tank.

Macro dead time - make sure you are making use of everything that you need to use and nothing that you don't need. Check here. Don't overlook things like special abilities from AAs or traditions and master smites. Make sure you're not gimping yourself by overwriting spells that have a quick recast but long duration. Make sure you use your debuffs. Don't hold back on temp buffs.