I've been trying Mega Zygarde a lot, and while I don't think it's top tier... it feels like a puzzle to solve because when it pops off, it feels like it goes into this sicko mode where it gets like 4-5 KOs before going down!
I agree that Endure and Zygarde-10% is the best way to use it. I've been using 44 Def / 252 SpA / 212 SpD mainly because with Reflect up, Zygarde-C can actually survive Garchomp AFTER Power Construct which is sorta nuts lol. For the record this is the set I've been using:

Zygarde @ Zygardite
Modest Nature
44 Def / 252 SpA / 212 SpD
- Core Enforcer
- Earth Power
- Dragon Pulse
- Endure
I use Dragon Pulse mainly just for quick Garchomp kills because Nihil Light feels like overkill lol
So while it's clunky, the best results I seem to be having with it is pairing it with an "offensive dual screener". By that I mean, I've been using a Life Orb dual screens Delphox as a way to try and mess with ice types and metagross lol. I feel like a dual screener that goes all in doesn't feel great just because it makes the team even more reliant on Mega Zygarde, having a dual screener with some bite feels like the play.
This is the Delphox set I've been using. It's been XS but I might switch to Alpha for the bigger Heat Wave. It's nothing crazy but it feels like a rare dual screener that is very capable of actually getting KOs on its own when Zygarde shouldn't be coming out. I should note too contrary to what you may expect, I often Endure and then Mega Evolve Zygarde BEFORE getting dual screens up in certain periods of the game. Idea is, with all the invincibility frames, I then switch Mega Zygarde out to the dual screener so position myself better!

Delphox @ Life Orb
Modest Nature
252 HP / 252 SpA / 4 SpD
- Heat Wave
- Psyshock
- Reflect
- Light Screen
I've been a maniac and even tried it with TWO dual screeners (a small size Metagross with Heavy Slam / Ice Punch and Expert Belt specifically to make it less of a heat wave collateral damage bonus target lol, you'd be surprised how much smaller non alpha metagross is!). It's "worked", but currently I'm looking into a back up offensive option to use with dual screens Delphox. I've also tried LO Slowking to give an idea, and Weakness Policy Froslass, but so far the best "offensive dual screener" with Mega Zygarde has def felt like Delphox.
Now as for regular Zygarde... I think this mon is actually better than you may expect! However the set is gonna seem... really odd, but its a set I HAVE seen other folks win with before!
Zygarde @ Life Orb
Adamant Nature
44 Def / 252 Atk / 212 SpD
- Land's Wrath
- Thousand Arrows
- Outrage
- Extreme Speed
I realize this may seem absurd: really Life Orb on a base 100 Atk mon? The secret sauce however is... Land's Wrath of ALL things! Gamefreak must have felt bad for this move, because in this game, Land's Wrath is a fast and massive physical Earth Power! Its fast and big enough that I've found if you keep Zygarde in the back, it can legit snipe quicker than a lot of folks can switch out. Even folks that switch out in time are often STILL getting hit because Land's Wrath is just weirdly massive. Cool thing is, since this Zygarde is more about stealing kills with snipes, the poor movement Speed and "I MUST POWER CONSTRUCT" don't feel like as much of a hindrance.
Extreme Speed may seem silly, but it lets Zygarde actually sneak up and kill Sash Garchomp pretty effectively and gets around that sucky speed before you Power Construct. I did try Thousand Waves, and while "EQ that traps" is cool, I think that might be best left to Sash Zygarde-10% because Extreme Speed stealing sash kills is too useful.
From day 1 so far, I actually really don't think Zygarde is a Diancie situation. Don't get me wrong I think Xerneas and Yveltal are still the best two restricteds and are easier to use, but between LO Zygarde being a surprisingly good kill stealer and Mega Zygarde... sometmes going nuts, I THINK Zygarde is a puzzle that's still worth solving right now!