That must be why my rejects dropped to nearly 0%, stayed around 0.8% and the pool's hovered around 5.3% over the past 50 minutes! I was wondering what happened, lmao! Very nice, Terk, very nice.
And I have some good news!
I finished bugfixing another 3rd party stratum server to replace the previous one. I also rewrote it to be multi-threaded instead of single threaded (which is a big issue). I am testing it on some percent of the pool hashpower and results are very promising - number of rejected shares is significantly lower. I'll let it run serving part of the pool for an full hour and monitor logs and stats to see if no other problems arise and if nothing unexpected happens, I'll be switching the whole pool to it.