Can I offer you a nice tentacruel in these trying times?
Tentacruel @ Black Sludge
Timid Nature
Ability: Rain Dish
EVs: 120 HP / 152 SpA / 232 Spe
- Rapid Spin
- Scald
- Dazzling Gleam
- Knock Off
this stat spread lets it compress rapid spin(generally better than defog), checking urshifu since it's faster and being an ok check to spectrier in rain, or shaky check outside of rain,which is good enough since your blissey has shadow ball. Good thing about it is that both scald and knock do good damage so spectrier can't do it's disable shenanigans, which means it also does well into the spectrier set that counters your blissey. Also this thing has mad regen in rain. In your team I'd prolly suggest to try this in place of tapu fini, and then you can maybe replace buzzwole with another abuser,since tentacruel also resists fighting. I also think that in general rain should probably be a bit more offense oriented than your team, so tentacruel compressing some defensive things that buzzwole and fini offer to make more room for abusers would be more along the lines the most common process of building a rain team.
some relevant calcs:
152 SpA Tentacruel Dazzling Gleam vs. 0 HP / 4 SpD Urshifu: 344-408 (100.8 - 119.6%) -- guaranteed OHKO
252 SpA Spectrier Shadow Ball vs. 120 HP / 0 SpD Tentacruel: 121-144 (36.5 - 43.5%) -- 14.6% chance to 3HKO after Rain Dish recovery and Black Sludge recovery
152 SpA Tentacruel Scald vs. 0 HP / 0 SpD Spectrier in Rain: 156-184 (45.7 - 53.9%) -- 3.1% chance to 2HKO after Leftovers recovery
0- Atk Tentacruel Knock Off (97.5 BP) vs. 0 HP / 0 Def Spectrier: 142-168 (41.6 - 49.2%) -- guaranteed 3HKO (knock loses power after first knock, but I think it's still enough for a 3hko, if it ain't because of lefties run hasty instead of timid i guess

)
The only way spectrier wins in rain is if it presses an attack every turn. If it tries to do sub disable stuff it straight up loses. It can also cover for blissey decently if blissey teleports into it on an urshifu switch, or switches it in on an expected trick, only for an opponent to get black sludged instead.
EDIT: now that I think of it, just as tentacruel covers very nicely for blissey, so too does thundurus cover very nicely for it, what with being immune to two of the types tentacruel is weak to. The only thing that really threatens this core is a big fat psyshock.