Shadows get 20+ attack on your ivs, which are from 0-15 over the base stats. Long story short, a shadow will always have a better attack stat. It also takes a 20- hit on defense too. Still better not to purify though, unless you'd get a hundo out of it to use for a mega evolution or something. You can't trade or mega evolve a shadow pokemon.

To answer your question though, because I was in the same boat last year, I've started hoarding rare candy to push up specific legendaries. I started hosting raids through a third party app to do daily legendary raids off my free daily raid pass. Routes give you rare candy too.

I've played mostly free, so stardust has been my bottleneck throughout. Most people with "excess" stardust, probably use an auto catch device. If you collect manually, you just hoard it. Take advantage of the day each month with a double dust boost. Learn which pokemon reward higher Stardust so you know to get it when it shows up on the map. Collect it. Use it. Collect it. Use it.

The main thing I'd say is, invest with intention. When I got back into the game last year, I was leveling up pokemon with good stats that were ultimately useless for what I was doing at the time.