However, now, it is backfiring and popping like nobody’s business at low revs (maybe high revs too but just cant hear it). If I let go of throttle, it eventually drops the revs and dies
The most common cause of problems after carb swaps/servicing is air-leaks being introduced so check again for them.Your above statement leads me to to this along with cable routing and of course Carb Balance check.
Also the 2nd hand carbs need some fuel cleaner in the fuel in case they are gummed up if you didn't strip and clean them.
As for Carb syncing it's not that hard with a Carbtune2,
https://www.carbtune.com/inst.html
I've done loads of carbs with mine.