What to do with dust ...

Have you maxed out the mons you are using?
Is there a particular monster that may not be relevant but you like it for whatever reason? Power that up.
Do you have any monsters with legacy moves?

you could take some jumpluff, medicham and azumarill to their caplevel for great league and make your friends happy!

Trading Dust for Shiny pokemon is possible... sorta.

Just advertize in your local discord/chat that you will trade Level 40 X for a Shiny Y.

Anything not catchable in wild is a dust savings at level 36-40.

Figure out which of your Pokémon have the highest Ivs by using a Pokémon iv calc. Then from their I would invest in powering up and buying moves for pvp and what not my opinion only. Idk what other people do.

Unlock a second move on everything you own for PVP!
Might I recommend double-legacy D'nite to get Hurricane before it gets banned for being so OP, QuakeBeam Groudon isn't too shabby, have all of your SB Mewtwo to have each of the current elemental moves as a secondary, Earthquake Metagross is quite the friendship destroyer, SA Lugia paired with just about anything, and turn your legacy SA Moltres into multitasking GODS, just to name a few I know first-hand are the absolute crème de la crème