Same dilemma here...

Honestly I don't feel like med threads are worth it. Better get those from monthly event instead. What I'm doing this month is getting 1050 keys and spending the rest on myoboku's gifts (I used to get med refines before). You may not get many myoboku's gifts this way (200-300 a month maybe), but I feel it is risky to ignore the cave key rebate and go all out on myoboku as you might fall behind on initiative.