This was really cute and I love this subversion of the usual "childhood friend was actually a girl" thing, buuuut it couldn't carry a series. This is about as much mileage you can get out of just this joke. Still, loved it for what it is, I've always wanted to see one of these end up this way.
The bisexual tag is for when it's plot-relevant that a character has both types of relationships I think, but wow these arguments that imply that characters being bisexual would make it less yuri are... hm... I mean, comp het is a thing, you could make an argument without throwing bisexuals under the bus like that.