Mega Delta Venusaur is an excellent Calm Mind user, but changing Mega is your choice. The set I could propose is, althoug the Nature is not that important, the Ability is mostly useless here:
Venusaur-Delta @ Delta Venusaurite
EVs: 240 HP / 244 Def / 24 Spe
-Hidden Power Fire/Psyshock
-Moonlight / Focus Blast
If you want a different Mega Evolution, the choices are very numberous. Mega Delta Typhlosion is terrific under New Moon or Rain, Mega Kangaskan is absolutely BROKEN, the two Mega Charizard are awesome as a Dragon Dancer for Charizard X or Wallbreaker for Charizard Y, Mega Gardevoir can break most walls, Mega Lopunny is very good, Mega Typhlosion is hard to stop, Mega Ferraligatr is a dangerous sweeper, Mega Flygon is both fast and has increadible sound based moves and Mega Delta Metagross is very strong and quite fast. There is so much more options, but those are my mentions.
If you go to the Friend Safary, you can find some Servines that have the ability Contrary. Thus, the Special Attack boosting Leaf Storm is a possible option and is what I would recommend. The player " Roachy " has Servine in its safari. Little tip, use a Timid Synchronise user in order to have more chance to have a Timid Nature. This is my suggested set:
Serperior @ Leftovers
EVs: 56 HP / 200 SpA / 252 Spe
-Glare / Leach Seed
So about what to remove… It depends.
Of course, you could always demand me to make a perfectly crafted team that remove 5 Pokémon from your party but who am I do decide that? Always keep in mind those are suggestions and not orders.
Your Delta Roserade is good, I do perfer to use Noctem than use moves to setup weather, but it’s fine. I don’t like having both New Moon and Mega Delta Venusaur. It is viable due to the limited weather turns, but still.
I think that Swampert could be switched. There are a lot of good water and ground types out there. If you have a Timid Delta Hydreigon like you said, that’s a jackpot! Good water types out there are but are not limited to: Starmie, Delta Haxorus, Protean Greninja and Gyarados. Moreover, I have crafter a Gyarados Set that allow you to single handedly defeat 1 and an half member (Kyla gets annihilated and Eduard has a difficulty).
Delta Frostlass, without its Mega Evolution, has some difficulties. Competitive is not bad per say, but without its main reason of being used (Defog and Intimidate) being very proeminant, it is less usefull than it seems during Late Game. If you are looking for good Fire types, there are few good choices. Mega Charizard (X and Y), Gale Wing Talonflame, Deta Chandelure, Regular Chandelure, Speed Boost Blaziken, Volcarona, Infernape and Mega Typhlosion are your best bet. Delta Liligand can work too.
I personally don’t really like Magnezone without Magnet Pull. What I would propose you is your Detla Gallade, a Choice Scarf variant:
Delta Gallade @ Choice Scarf
Ability: Volt Absorb
EVs: 252 Atk / 4 SpD / 252 Spe
-Brick break / Stone Edge
This set will allow you to hit as fast and as hard as possible. Choice Scarf allow it to outspeed Pokémon usually faster than it and beat them. It can beat quite a lot of Dragon types that are very popular during the mid-late game. For Steel types, Delta Luxray, Aegislash, Scizor, Delta Haxorus, Delta Typhlosion, Bisharp, Excadrill (use Mold Breaker if you don’t have Sand Stream in your team) and Lucario.
Tell me if you want to change anything, I can help you just a last tip: always carry Taunt for the elite 4.