The only low-key cool thing about it are the relics you can find and then upgrade.
Some of the results are cool. The process isn't. You receive a random item, spend resources + tying up your diplomacy council for a week on it. Then maybe you get a choice of items immediately. Maybe you get an intermediate step. Maybe you get a choice between 3 weapon types, none of which any of your characters can use. Maybe you could have used it if you had picked wand in the intermediate step, but you picked armor and none of them are useful (which is a fun thing to realize after 2 weeks of in game time).
Sword Saint makes an insanely powerful gish and with the Lich weapon abilities you get
Sword saint can't merge spellbooks, since it doesn't have full arcane casting.
I don't want cheese, just a straight class/prestige class build. Game is already too easy.
There's only 3 classes that can merge spellbooks with lich: arcanist, sorc and wizard. And you won't get CL25 on a lot of spells without merging. And sorc is the clear superior choice for a gish, since it has medium bab. Then you can take eldritch knight (-1 CL) or dragon disciple (-3 CL) as you please. Lich adds 13 CL in total iirc, 10 from mythic levels and 3 from mythic ability.