What's the right fix for this?  It gets painful to shoot.

I’d start with the hammer.
My WASR 10 came with a Tapco trigger, the one with the squared off back on the hammer. Bad trigger slap and my bolt was peening on the back.
You can grind it down. Some don’t recommend that as it might affect the hardness of the hammer but IMHO better than peening.
Best bet is to put a proper hammer in it, or use it as an excuse to replace the trigger group.
I replaced mine with a RAK-1 and was worth it.

change out the FCG. The triggers that Century used for SAR-1s were BRUTAL. the disconnector tail was too long and they caused trigger slap.

many triggers out there that will work. I prefer the Romy Semi auto FCGs from Apex... good romy trigger for a good gun.

https://www.apexgunparts.com/ak-semi-auto-trigger-pack-w-hammer-new-romanian.html

you can use one of the myriad of USA made fcgs too

Century Rak-1
Fime
Fime Enhanced
ALG (I don't reccomend the ALG triggers though. people love a light bougie trigger, but it's almost too lite and it's over hardened and beats up carrier tails. most over rated AK trigger)

The fix. Worked for me

http://www.novarata.net/Linx310/slapfix.shtml

Cut the tail off the disconnector.  Done...