Imo this team is very lackluster. Not to mention, Primarina actually has a very bad set here as well. (Something I personally wouldn't run.) There can be some pretty good synergy going on here, but it really depends on being able to spot niches.
I already said it, but I might as well say it again: my biggest issue is the Primarina set. It's not uncommon/unusual to run Primarina with specs, but Primarina does not have enough utility to actually utilize the choice specs as efficiently as other Pokemon. Recently, they introduced the Metronome held item. You can use that to your advantage instead of choice specs. Primarina also has access to echoed voice which increases in base power the more you use it. See where I'm going? Pair this with metronome and you get double base power boosts, and just after two echoed voices your echoed voices will have 100+ base power without stab. By the third or fourth, 200 power without stab, with stab is 400. Now imagine that in rain where water types get boosted. Primarina is also definitely not outclassed by Tapu Fini, Primarina can get much stronger as the battle progresses while Tapu Fini does not have the same wall breaking power Primarina has. Just for reference, I was able to take out a ferrothorn, toxapex, and hydrapple with an echoed voice while hiding behind a substitute. Chansey and Blissey were a bit more difficult, but not impossible with enough Calm Mind setups. Mind you, Ferro, Hydrapple, and Toxa all resist water type moves. I would usually run Echoed Voice, Draining Kiss, Calm Mind, and Substitute, but in your case Calm Mind and Draining Kiss are replaceable. Substitute is also replaceable, but when you're a tanky Pokemon who is gradually building up power with Echoed Voice, it's suggested. Draining Kiss also benefits from the Metronome which allows for more recovery and more damage, thus why I used Draining Kiss. Substitutes were never an issue whenever I had draining Kiss, but you're free to decide. I often ran this with Trick Room instead of rain, but you're free to choose, again. The priority lies in what you want to center your teams around. I build off balanced teams all the time because I refuse to use weather based or terrain based teams, Trick Room was a one time exception when I got too bored. Also because I only like using teams which include Pokemon I like, not too much of a meta slave. I often ran Primarina as a damage sponge to better balance my team. (Never rlly worked out tbf, it only ever hard countered rain teams.)
Gyarados was an optimal choice, but you would want to use Swampert in a rain team. Especially since you're considering to run EQuake.
Other answer also suggested a better Jirachi set, their answer was great.
Xurkitree can also definitely be better, look at the other answer for that better set. There are also better swap outs if you wouldn't prefer that.
Garchomp is... an odd pick. You can also go through the safe route with Tornadus, but I personally had a blast using odd sets in rain. Never favored rain teams, but I did once pair Goodra with Archaludon. The priority was to make Archaludon a sweeper, but it was never fast enough to steal kills and just two strong fighting type moves took it out. Then came Goodra, whom I originally used as a rain setter, who clutched. With Hydration and EXTREME special bulk, I was able to spam rest and wake up the next turn without taking much damage. You can pair this with Sludge Wave, thunder, and draco meteor and you've got yourself a decent damage dealer with extreme tank capabilities. If you'd rather it be some kind of stall Mon, that always works too. Toxic, protect, and another move of choice is great. I personally ran Sludge Wave cause I often played Nat Dex and Tapu Lele is an extremely common Pokemon within that format. Goodra was able to tank about a good three moonblasts before being taken out by a specs lele, but that leaves you with two free turns to deal damage and spam rest.
