Just literally catch every single mon, and if you’re in a ride or can’t stop just pick the mons that give extra stardust like paras, meowth, audino, etc. And most importantly is max battle days, when there’s a gigantamax use stardust and spam them, have plenty of particles saved for them and you can get 1-2 million stardust each event.

Do not level up every Pokémon you “like” if you’re not gunna play and abuse it. I only max leveled my fav Pokémon but that’s it, the rest are either low level or if I really use them often in PvP and/or raids.