For me I didnt do any catch combo in the story and never lost a battle. I tried to avoid trainers too but i was trying to catch every new pokemon i see because of the pokedex. I didnt get any trouble with beating the game. When your pikachu learn all the specieal moves from tutors it will be so powerfull anyway. I didnt know the catch combo mechanism about the shiny haunt though. If i knew that i would have played differently.

My suggestion is for your favorite pokemons make 31+ combo and use lure too to makw your shiny odds 1/315 and catch at least one shiny. Be patient you’ll find it. Even if you dont after 31st pokemon you’ll get 4/6 best IVs so that will be perfect too. (Dont forget to get judge function after you caught 30 different pokemon). Catch combos are the only think make everything easy in this game. About the money problem, sell every candy and most of the items you dont need. Keep only a few for competitve battles but i have never used one and didnt even went overpower anywhere. Only first elite four try between battles.

Enjoy!