I have a severe case of Last Elixir Syndrome, and other than the item slot increases, I haven’t spent a single bounty token or dog coin, and I’m now at level 60. What is the most effective use for each?

Bounty tokens for implants (the bounty traders can be found in the settlements/villages)

Dog coins for power fruits (bought from the gold Black Marketeers found at ruined churches)

All NPCs are catchable, and shopkeepers can still sell you items if you capture them and/or place them in a base. It's actually a big meta strategy so you don't have to constantly go out to farm resources/currencies.

ETA: implants are great for getting your pals OP without having to spend time breeding specific traits down. Use with the surgery table. And the relics you get within the World Tree can be used with the ancient recycler to get those special traits (and other WT specific drops)

Go check out a church ruins, or speak to a bounty officer in a town.

Sure but like, are the hats worth it?

The hats are just for fun. Their armor stats suck, but you can transmog your armor with the Dresser furniture.

It's the skill books, star fruits, and implants that are.