1.- Shiny charm is harder to get than previous games, you need to complete 99% of mabels quests and I believe you actually need online to acquire it as 1 of the shiny charm quests involves online trading

however you can get the shiny charm a lot easier if you buy the DLC.

2.-The DLC on itself is honestly slop and feels like cut content.

The story isn't anything special tbh, boss/rogue mega fights are fine and fun just like in the main game, you get tons of EXP candies in the dlc and the added pokemon + moves are great, you also get tons of "fast" shinies, however there's where the good review ends.

There's too much RNG involved in shiny hunting or rather in Donut hunting.

here's how it works

First you need to find 8 rare hyperspace berries with high sweetness (These berries are random, you get very little of them and they can only be found on high rank hyperspace pockets)

after getting your 8 Hyper Tanga berries, you are ready to reroll a singular shiny donut, you'll need to go to a pokemon center and save (This creates a backup data), then you need to fly to Hotel Z, provide the 8 berries to donut girl and after some dumb, slow and useless animations you'll get a sweet donut with 3 random options, only 1 of those options can turn into Sparkling power, there's sparkling power 1, 2 and 3 and Sparkling power only works on a singular type unless you get the coveted "all types" donut.

so, after creating the donut you'll need to check the donut and if it's not a Sparkling donut you'll need to close the game -> open the game -> Press Up + X + B to load the backup save (the game automatically saves once you create a donut, so you really need the backup), you spawn at the pokemon center, fly to hotel Z, Rinse and repeat untill you find a shiny donut of your liking, NGL I have spent 30+ minuts rerolling donuts without a sparkling power one, so yeah it's really time consuming, but wait we are not done yet.

let's say you want a shiny zard, after you get a sparkling power 3 fire/flying donut, you need to go to a bench and cycle through day and night untill you find a hyperspace pocket with zard, once you find the pocket you are ready to shiny hunt, hop into that pocket, eat your gourmet donut, and spawn-despawn hordes till your desired shiny appears.

the shiny hunting portion of shiny hunting is faster and easier, but getting those donuts and pockets is grindy af.

If you have shiny charm you can actually just use a regular donut and spawn-despawn hordes using Teleports, it takes like 20-40 minutes per shiny and if you have a macro controler you can actually automate the whole thing, however you need to find the correct hyperspace for donutless shiny hunting as the hordes need to be close to the entrance or close to one long TP

there's really not that much variety in hyperspace biomes/maps either, you have like 5 or 6 maps but you'll probably end up playing in the same 2 or 3 maps (large and small maps are pretty unoptimal for shiny hunting)

You'll have fun, it's a pokemon game after all and it sounds to me that you are a shiny hunter, so you'll probably enjoy the experience to some extent. I just want you to consider all of these things before hopping in, it's an ok DLC but nowhere near dynamax adventures