Yes we have magic and dragons etc, BUT that doesn't take away from verisimilitude.
Thor's hammer returns. All hammers do not. (You have to spend a little more investment/magic to play specialised heroes).
Water does not flow uphill.
Trees produce leaves.
Javelins do not fly around corners.
Boomerangs do not return easily, and certainly not in confined spaces or when they hit something.
Barring magic, these things are consistent in my settings.
To try and help with stats (and how I would use them):
I would go with club (though the large Koori hunting boomerangs used down here could easily be bumped to a d6 damage - I didn't realise a club was so low). Those ones never return though.
Smaller ones; d4 damage; return on a miss (if there is enough space to either side equal to the range of target would make sense. eg: throwing at target 40ft away, there needs to be 40ft of open space to either side b/w you and target).
If you want boomerangs returning in confined spaces, that is good for you, but it is not how a boomerang works. If it is to return it travels in a large loop, and often circles above you again before the catch. Getting to come back to where you stand is extremely hard. Getting modern ones, crafted specifically to return to land within several feet of you is a skill. As stated, could even go with requiring an action/reaction to catch (possibly on next round for long range).