A different skill ticket question

So I'm sitting on 16 skill tickets, but I've reached a point in the game where my tsums are maxed out at SL6, save for some limited and event tsums from 6 months+ ago when I couldn't max them out.

Are there tsums that are never going to be available again that I should just spend them on? Should I treat them like premium tickets and spend them on whatever new tsums come up and save myself 30,000 coins? Something else?