1/2in of chain sag seems like a lot to me.
I typically get the chain as tight as possible. I’m sure that’s wrong for one reason and another. But seems like things loosen up on their own.
I’ve noticed my fixed gear clunking- but then I also noticed the chain was loose/sagging. So I’m assuming I need to tighten the drive train to fix the noise.
any chance you have a different freewheel you could try?
1/2" slack in a fix gear/single speed chain is normal and healthy. It should never go tight. (Pedal several chain revolutions to make sure. Both chainrings and rear cogs can have asymmetry. Until you line up both you haven't seen the tightest the chain will see.)
Clunking in a fix gear with chain slack - probably less than smooth pedaling style. When you get lazy and pedal slower than the bike, the bike will remind you quite abruptly. And with a clunk. Incentive to up one's level of pedaling grace. Says he who has ridden those things almost 50 years and more than half way to that orb in the sky. Still gets visited by 'the clunk" every once in a while.