The best way to grind money is to invest in a Pickup team.
Get 5 pokemon with Pickup and as you grind your team rotate the member out while keeping the other five slots filled with you Pickup mons. Every mon has a 10% chance to pick up something, be sure to keep them within 5 levels of the cap because you get better stuff depending on how high the level is.
The earliest you can get Pickup is from the Meowth on top of the Chrisola Hotel. Simply save before hunting for a Meowth, as soon as you catch it be sure to put it in your party to check it has the right Ability, if it doesn't just hit F12 top reset. GourmtTreats are quite a common pick up in low levels and sell for about 1k, then there's Nuggets; plus you get Great Balls, negating the need to buy Pokeballs early on.
Here's a table for reference, but I don't know if it's up to date with v13 exactly.
https://rejuvenation.fandom.com/wiki/Pickup_Table
And here's a list of Pokemon with Pickup
https://pokemondb.net/ability/pickup
So on Terajuma you'll be looking for a Trumbeak, just don't let them evolve.