Brice is a two pitch pitcher. Those guys typically don't fare well as starters unless one of those pitchers is a very plus pitch. Brice does not have a plus pitch.
It's modern baseball. Relievers just don't seem to think they can pitch an inning (or part of one), sit down for 10 minutes while their team is batting, and then go back out and do it again. Part of it is the way they are used in the minors. The one's who can do it are generally recently converted starters.
Personally, it's something that ought to be worked on in a normal spring training. Pitch a guy for an out or 2, sit him down and send him back out again. That could be done in an exhibition game or in a simulated one. This year was not a normal spring training, so I'll give a pass on it this time around.
As for Brice, he looks like he could be a fairly reliable middle reliever, but he's been up and down for 5 years seasons for a reason. Three decent games does not warrant an expanded bullpen role.