OU team

You'll want Fire Blast on YZard, and then you can get rid of Air Slash for something like roost or earthquake since sun-boosted Fire Blast ALWAYS hits harder than air slash, under all circumstances but two. Because of the way the types interact, it is only usefully stronger against fire/fighting pokemon, i.e. infernape since Blaziken and Emboar are never seen in OU. It's also stronger against water/fighting, but YZard has Solar Beam for them. Even when fire blast is neutral and air slash is SE, sun-boosted fire blast is still stronger.

Politoed and YZard on the same team don't seem like the greatest combo though, especially since nobody else on your team cares about the weather except Azumarill for rain, and Excadrill actually prefers no weather since it is weak to both fire and water attacks. I would say consider a different mega instead of YZard, perhaps Mega Blastoise so you can still have a powerful special hitter but one that benefits from the rain. You do risk having too much of an electric weakness, but Excadrill more or less mitigates that especially since it's also neutral to HP Ice. Maybe consider Assault Vest on it since you don't have any support moves anyway, and you have Staraptor (and Zard unless your replace him) to avoid earthquakes or walls like Sliggoo and Politoed to tank them.

I agree, Sliggoo wants the rain, which is why I think you should keep Politoed and replace YZard with something else that wouldn't interrupt your rain.

Dual weather isn't so bad, but when one of your weather only helps one pokemon and actually hurts some of the others (Azumarill and Excadrill) it might not be a good idea. It also cuts off Sliggoo's free recovery if YZard switches in while the rain is still going. You could even switch YZard with something that appreciates the weakening of fire moves, like Mega Scizor, though that might overload you too much with physical attackers.

You might also consider damp rock on Politoed. It is a trade-off for the loss of leftovers, but consider that having to switch in more often could lead to more hazard damage (or taking more attacks on the switch-in) which could offset the leftovers recovery.

Mega Ampharos might actually be a good idea, as it would provide even more defense to electric moves. But, you'll have to worry about having two fairy weaknesses which may or may not be a big deal to your team. I'd recommend C+Ping the team to a new team in Showdown and trying out some edits. Mega Ampharos is definitely something to try. He plays primarily as a bulky pivot, but a slow but strong thunder set could work too, especially given its hefty defenses.

I'm not sure for agility mega ampharos, as several things would force switch it out, but what about thunderwave>Agility?

This team is good, he smacked my ass with it not too long ago.