My understanding is your back elbow should be parallel to the ground. If you look at a picture of Ernie Banks, for instance, this is exactly his stance. I play softball (senior leagues) several times a week. I generally hit the ball hard, usually line drives. I’m a good lead off hitter. My back elbow is probably 15 degrees down. If I were to adjust my elbow up to "parallel " would I be likely to hit the ball any differently?
Disclaimer: I am not a hitting coach, by any stretch of the imagination. 
Decades ago, I bought the book “The Art of Hitting .300,” by hitting coach Charley Lau (who gained fame for helping to develop George Brett as a hitter).
In the book, Lau had a lot of photographs of various hitters whose approaches he admired, and he pointed out that there was a huge diversity of batting stances – that is, how they stood as the pitch was thrown, how they held the bat as they waited for the pitch, etc. But, Lau noted that, once they began their swing, and moved the bat into the “launching position,” they were all remarkably similar.
It seems to me that Lau would have said, “I don’t care so much where your elbow is when you’re waiting for the pitch – it’s where everything is when you’re swinging that’s important.”
It used to be standard practice to rest the bat on your right toe (if right handed) when in your stance but modern batsmen tend to lift the bat off the ground totally when preparing to take strike. The stance was first brought to prominence by an English captain who couldn’t bat for shit.
Don Bradman used to rest the toe of his bat between his feet and he batted .996
Seems how you prepare is of no material importance provided you are in correct position once you begin to swing.
If nothing else it shows Charlie Lau knew his onions.
Stance doesn’t really matter as long as you can get your swing to where it needs to be when it needs to be there.
And if you’re hitting hard line drives that’s a pretty good sign.
One caution, there is a difference between baseball and softball swings, primarily because baseballs are going down and softballs are going up (without spin). It does make a difference in swing angle, but it sounds like you’ve got that covered.
Ah, the ol’ elbow up or down debate. It’s not much of a debate anymore, a huge majority of MLB guys start with their elbow up, as the OP describes.
My best friend is a D1 college baseball coach and we have many philosophical discussions on baseball, including this very topic. I coach at a much lower level (little league) but I said it drives me crazy when parents only advice to their kid is to “get your elbow up.” It doesn’t even really matter. For some kids, elbow up in the stance does more harm than good because their just going to drop it down again and uppercut. My friend says, “but it’s the right way if they want to hit with power.”
He said, “show me one MLB hitter who has his elbow down.” This guy is a Cubs fan so I took great delight in the fact that my example was none other than Anthony Rizzo. Rizzo is quite an anomaly as a power hitter who does this. Typically, I think of guys who slap line drives like Rod Carew or Wade Boggs when I think of elbow down guys.
Bottom line, do what works for you.
‘Do what works for you’ applies to batters who have something that works. I don’t know that elbow up makes a superior stance but a lot of batting coaches have this notion. If it’s not something that works well in improving what works for a batter then I think that would be noticed.