That's a good point that I hadn't thought of. Spell Master would make it really easy to get 3 of the 1 round/level buffs to 24 hours. It's a little more limited than arcanist in terms of the potential number of those shorter buffs it can get to 24 hours, but realistically how many of those will you need to cast. Thanks!
Merged lich/angel spellbooks are higher caster level aren't they? becoming caster class + mythic rank so you get to rank 30 by being a merged angel/lich, Quarterstaff of the war mage gives +2 caster level and now since the DLC's are out the Robe of the Seven Sins gives +3, so those combined is 35 off the bat which should be able to do any spell with the extend metamagic.
Indeed, the caster level for the merged lich/angel spellbooks brings your MC to caster level 30. Since you only need to get to level 25 to get every extended buff spell to 24 hours, this is the simplest way to do that. This only works for the Main character though so for everyone else you'll need to rely on gear and feats/mythic feats. The robe you mentioned is a great way to get those caster levels since it gives +3. However, The Quarterstaff of the War Mage doesn't give +2 to caster level, but instead gives +2 to DC saves for spells that are cast and +4 to caster level checks made to overcome spell resistance, so sadly it is not helpful for this purpose.
