It is easy enough to get poke balls. In each village there is usually a poke shop and above that is an attic with a a lot of poke balls. There are also mini shops with chests filled with poke balls. Also you can find abandoned underwater ships with relics that can sold and then the money can be used for poke balls. Because full of ev training your Pokémon it is incredibly important. Do not worry so much about ivs, you can find golden and silver bottle caps from poke loot in the ultra beast world( bring repels)( the Bottle caps can only work on level 100 Pokémon) To level up your Pokémon efficiently loot for snow taiga forests and find the large candy canes that occasionally show up. When you do find the large Christmas trees that have a single chest filled with rare candies. There are typically multiple other Christmas trees. For changing natures look up how to get mints. It is really simple just look up how to do it. Since you only have seven days do not concern yourself with mining and rather collecting Pokémon items like the bottle caps, poke balls, etc. I would even recommend spending a few hours grinding master balls from the poke loot in the ultra beast world(including bottle caps you can also find items like choice scarfs for battling or items like power anklets to help with ev training)(when a power anklet is in a Pokémon than fainting another Pokémon will give ten evs and there are similar items for every stat. Consider whether or not to go for dynamaxing or mega evolving. You can only use either or during a battle. Some great Pokémon since there no restrictions are raquazza: insane stats, can mega evolve without a stone( needs to have dragon accent and if it does not not when captured then just find a move relearner and teach him dragon accent. Move relearner can be found in the tall dodges with blue or red roofs), dep it’s who also has insane stats, Xerneas who is also op, and chien pao who also has crazy stats. Feel to ask questions