Would it make sense to use a closer if one of your better pitchers has a bad sta/dur combo? Like if you had a sta/dur combo of 20/40 or something around those areas. You only have a certain amounts of innings out of that guy anyway, you could theoretically guarantee he would be put in higher leverage situations. I've been just putting all my better relievers as SUA, but I've considered putting one with a lower durability as closer.

Closer is, IMO, a lower leverage situation. Ninth inning, three outs, starting with no men on base. My better relievers are SuAs. My closer on one of my teams is what you suggest, 51 Dur 18 Sta. Mediocre splits, two pitches in the 80s. My other team's closer is basically a one-pitch guy, great first pitch and poor second pitch. They're really limited players, but they can get three outs more often than not.