Hmm. That looks kinda ok.
You don't
need the Special Defense investment if you're boosting it with Calm Mind, anyway. I'd take Dragon Pulse over Substitute every day since it provides a damaging move that is not Psychic. You could go for Stored Power instead of Psyshock if you feel like you can make opportunities for Latias to set up more Calm Minds. The only other option I really see is Hidden Power: Fighting if Bisharp is something your MegaLatias has to deal with (plus it gets you that unpredictability factor) but is more useful on a Life Orb set. Also, you're investing fully in Special Defense which will be further boosted by Calm Mind, leaving you very vulnerable to physical attackers. I recommend investing more in Defense like the set below runs.
I found this setup
Calm Mind
########
name: Calm Mind
move 1: Calm Mind
move 2: Substitute / Dragon Pulse
move 3: Stored Power / Psyshock
move 4: Roost
ability: Levitate
item: Latiasite
evs: 248 HP / 180 Def / 80 Spe
nature: Bold
Which looks fairly solid.
Edit: You said you feel like your set lacks coverage, you really won't be getting around this if you're running a Calm Mind set. You rely on the bulkiness and power you receive from the Calm Minds to dent the enemy and heal off with roost. If you want coverage then I'd recommend ditching the Mega forme and going for a Life Orb set. I LOVE Life Orb Latias even more than Latios and usually run
Latias @ Life Orb
Ability: Levitate
EVs: 32 HP / 184 SpA / 40 SpD / 252 Spe
Timid Nature
- Draco Meteor
- Psyshock
- Hidden Power Fighting
- Defog
Which I have on a team but I can't remember how I reached those stats lol
Or
Latias @ Life Orb
Ability: Levitate
EVs: 72 HP / 184 SpA / 252 Spe
Timid Nature
- Defog
- Draco Meteor
- Hidden Power Fighting
- Psyshock
Which is similar to most sets you'll find here at Smogon.