However, Sacred Fire is your only way of hitting Dialga and Jirachi for good damage. Even in rain, Support Dialga hates taking burn damage all the time. Jirachi does not enjoy burn either even in the rain. Skarmory hates Sacred Fire and can only wall you in the rain. This Ho-Oh set has so much Special Defense it will attract physical attackers such as Zekrom so Sacred Fire can deter them from switching it. Also, Ferrothorn can Leech Seed and out stall you if you drop Sacred Fire which is just bad.

Sacred Fire hits the Poison/Steel types who are immune to Toxic though. Brave Bird can still be the main option however. I am thinking about like this:

Brave Bird
Sacred Fire
Roost
Whirlwind / Toxic / Light Screen

You don't need Toxic for Latias anyway because you 2HKO with Brave Bird. Yeah Brave Bird main option for more PP, consistency and accuracy.

losing fire coverage is kinda bad but toxic / sacred fire kinda conflict so i would actually sooner slash it over sacred fire than BB. BB is also stronger in neutral weather and rain + hits ferro / forre neutrally

Brave Bird
Sacred Fire / Toxic
Roost
Whirlwind / Light Screen / Toxic

?

Just put Brave Bird in over Toxic then. Hitting Kyogre hard on the switch in is better. Burns + repeated Brave Birds will probably kill Arceus but Toxic nails Lugia which is fun :)

yeah, syrim has mentioned this before. i think it deserves an AC mention on the tank set.
i think brave bird is probably better than toxic

You might want to mention a Specially Defensive Ho-Oh set because it owns Kyurem-W. If you don't think it needs a separate set, an AC mention will do. In DPP, Ho-Oh had a 248 HP / 8 Def / 252 SpD Careful set with Sacred Fire, Toxic, Roost, and Light Screen. You can drop Light Screen for Whirlwind.

post in the normal arceus update when you have time; there's a lot of things i want to iron out.