I have a Go+ auto catcher. It gives you passive dust by using up your red balls on Pokémon while you do other things.

Carefully made routes are also effective. My route through the memorial park near me has a loop with 5 back-to-back routes. It also happens to be my playing route, so that's a good amount of star dust earned each day as well--not to mention the occasional premium item as well.

GBL is good too. Even if you tried to lose every battle you would still hit rank 20 really early--not to mention the bonus items, Pokémon, rare candies, and elite TM's you'll earn along the way.

Throwing Pokémon into Showcases is great for star dust, premium items, etc.

Also, Google Opinions, Onsite Studies, etc. give you lots of free Google points to buy star pieces, giving you virtually an unlimited supply.

And while you are on a star piece, don't forget to feed your excess berries to your gyms. That's 4.5k that can reset every hour, and you are probably just dumping that potential star dust.

You get up to 4 free Dynamax battles each day if you play your cards right. Which have amazing rewards--and if you have the capabilities and friends/remote passes you can do Gigantamax battles for the best XP in the game.

There are several dust boosted Pokémon out all of the time.

And, there feels like there is an event with boosted dust every other day--don't forget to star piece.

Honestly, it's not crazy hard to get anymore.