I like your house rules. Especially the Luck rules. I like the Luck roll to avoid the cost of spellcasting. Is a 12 on a Luck roll a crit fail? What if you critically fail the spellcasting Luck roll? Double cost? Roll on a Spell Fumble or Corruption table?
Recovering Luck too. Your art and creativity, plus leisure time, slant is a good one. My favorite Luck recovery mechanic from any game is DCC Lankhmar, where carousing restores Luck. Very relevant to the fiction.
personally with a double 6 critical failure on the luck dice, I would maybe make them roll on the OOPS table and pay the stamina cost.
if the spell was to like boil water for soup I’d probably not do the OOPS roll, but if it was to like curse someone’s bloodline or make fire rain from the sky then they can roll oops too.
i kinda think of magic in Troika as like weaponized serendipity. how much are they asking of serendipity? is the question I ask myself in those sorta situations