>But i couldn't get the boomerang blueprints because of the
>spikes.


you need to get a certain ability on an amulet to get it, or use the assault shield. there might be another way to do it

>Now that I've finished once, i just need to keep unlocking things i
>guess? Are there any new areas to go to?

some of the areas have multiple exits and most of those are only accessible after getting all the runes. i think you're supposed to have all the runes except one after you beat the hand of the king. just keep playing and exploring and you'll find new biomes and weapons. i do recommend boss cell level 1 to expedite the unlocking process