Have you tried to PUG high level mythics? It's a crap shoot, and you're competing against a million dudes with better gear. Atm the best way to breach the 850s is with a bunch of friends/guildies who are active in the mythic scene. Without that avenue, it's...pretty dire out here.
But like I said, that's probably how it should work. M(M)Orpg.
Impossible to pass 850 without a guild, because the pugs are full of people that are over 850. You get what I'm getting at right? Those people are pugging too, and they clearly got the gear somewhere.
I don't disagree that the EASIEST avenue is to already have a group of friends, but being in a guild doesn't guarantee you that either, hell I don't buddy up to the only two tanks we have running these all day, I pug about half of my mythic +s. It's actually quite easy: Make groups yourself, or just spam request invites from all of them. It's not your ilevel that doesn't get you invited typically either, I show as 873 when I queue and I get declined plenty, it's simple math, there are tons of people looking for groups and the leaders can afford to be picky. So, again, just make your own. Or make some friends, having a tank or a healer goes a LONG way towards instantly fixing the problem. My guild gets me raid gear and a bunch of friends. It gives me a place to type "Anyone wanna do a heroic?" or something, but it doesn't mean I just 'get' groups on a whim.
I'm not claiming groups just materialize out of thin air, but I always find that they're a lot easier to get into or make than those who complain about them seem to act. Play well, make friends, get invites, etc. It's how social games can and should work. If it's as hard as you're claiming to have good runs, then when you do you should DAMN SURE be adding those people to your friends list, or they adding you to theirs. If you can prove you're valuable to a group they will remember that.
Honestly, the biggest advice in general to getting something you want in wow, is to take on an aura of leadership. It doesn't have to come naturally, I'm VERY shy irl, but I'm very confident when it comes to wow, and that wasn't always that way. It literally is just bred from within, but it is what gets you to leading groups of 5 people so that they remember you. Make sure you know what you're doing in a dungeon, which doesn't require research but only observation. If you can become confident that you know what you're doing, you can influence those around you to play better and you can talk your way into groups or situations that you desire with ease. I'm sure Real life is the same way, I wouldn't know, but it is one hundred percent true in wow. If I get in a 5 man I take the reins whether I'm tanking or DPSing or whatever, and because I know what I'm talking about (which people can tell very easily just from how you're typing and what you're saying), they will listen. Similarly, I find I get into Mythic + groups more frequently when I start whispering the party leader after signing up. Just strike up conversation and make it personal. Doesn't walways work, but it doesn't hurt.