Hello there!
This is a wonderful example of an offensive team and the synergy between your Pokemon is great. Anyways, I can see offensive rain teams comprised of strong water type attackers, such as Specs Politoed and Rotom-W, being a major problem for this team, as Starmie takes a ton upon switching in and probably won't be able to switch back in for the rest of the match. This can leave your team in a pretty grave state, since hazards will end up eating away at Darmanitan and Dugtrio and you also lose your main defense against rain teams. Just to back this up, Starmie takes 82.06% - 96.56% from a SpecsToed Hydro Pump in Rain, which can easily amount to an OHKO with Stealth Rock, while Specs Rotom-W's Hydro Pump does 82.44% - 97.33%. Surf from Politoed does a bit less, but still puts Starmie in a position where it can't switch in very often. Overall, I think using standard rapid spin starmie would really increase Starmie's longevity and also your insurance against rain teams. You lose a bit of offensive power, but Starmie is by no means weak, as it still has powerful attacks and a Base 100 Special Attack stat.
Starmie @ Leftovers
Trait: Natural Cure
EVs: 252 HP / 252 Spd
Timid Nature (+Spd, -Atk)
- Hydro Pump
- Recover
- Rapid Spin
- Thunderbolt
On to the next issue, Rotom-W has a field day with this team. While you have Focus Sashes on two of your Pokemon, everything on this team is taking heavy damage from either Hydro Pump [Magnezone, Mamoswine, Dugtrio and Darmanitan] or its Electric Attack [Starmie and Sharpedo]. Scarf Rotom-W is particularly threatening as it can outspeed Starmie and Dugtrio before they can attack it. Plus, Darmanitan can't even OHKO with its attacks, so it'll take nearly your whole team just to take this thing down. You mentioned a weakness to Latias as well, and I think that Ferrothorn is just the Pokemon to help your team out against these two major threats. Ferrothorn also grants you an additional defense against Rain teams, as well as Entry hazards, which are extremely helpful to your team, since it relies on sweeping with hard hitters like Darmanitan and Sharpedo. The Pokemon I suggest you replace for Ferrothorn is Dugtrio, mainly because the Pokemon it traps aren't really hindering to your team. Heatran isn't particularly threatening, as Starmie straight up beats it, and 4 of your team members can revenge it. Trapping weather starters isn't really significant for this team either, since you have no weather yourself. It's nice to have another trapper, but unlike Magnezone, Dugtrio doesn't really trap anything that opens up a sweep for one of your main attackers. Anyways, hopefully Ferrothorn works out for you.
Ferrothorn (M) @ Leftovers
Trait: Iron Barbs
EVs: 252 HP / 48 Def / 208 SDef
Relaxed Nature (+Def, -Spd)
- Leech Seed
- Spikes
- Power Whip
- Gyro Ball
You could also probably throw Toxic or Thunder Wave on here somewhere, if Gyro Ball+Leech Seed doesn't handle Latias well enough.