Keep (repurpose) or Toss

Keep as is.

BrightPowder

Keep

Air Balloon

I've never used it as the description makes it sound like an one time use item.
But it's an useful item. So if it really is one time use, I'd remove that and make it just a normal held item in that regard.

Lure Ball

Keep and won't change.

Great Ball

Keep and won't change! They look good and they work well on most Pokémon :)

How about a Heat Rock?

Never used the weather rocks, but I'll keep them as is.
Kinda feel like they could be united into one item though.

Zap Plate?

Keep and pretend to be the Goddess of Thunder

Black Glasses ?

Keep and pretend to be the Goddess of Cool

Elixir?

Keep and pretend the be the god of mixing
Is this a thing now? =o

Cornn Berry ?

I have never even heard of that XD, Toss.

Black Belt (which is also a Trainer Class XD)

Keep as is.
Though I'm temped to tweak it a bit by having it increase accuracy...

Sticky Barb ?

Keep as is

Blunder Policy

Never used it and it sounds niche... I'll Toss.

Sachet ?

Keep as is.

Dragon Scale

Keep, but change it so it functions like the trade evo items in Legends Arceus. No trading required, just use it as if it were an evolution stone!

Energy Root ?

Toss. I don't like lowering happiness, even if it's just arbitrary.

Eviolite

Keep, pretty solid item.

Thick Club

I'll keep it.

White Herb

It's fine, but I dislike using one time use items. I'd either make it buyable or reusable, but once per battle.

Muscle Band ?