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:
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
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.