Few things to try and help you.
First the emax "simon series" ESC's are actually running blheli, they just called them "simon series" because at the time they were released simonk was more well known and common than blheli. But...it's not a normal version of blheli, emax hacked it up and modified it. You can reflash them with real blheli...but they're really not great ESC's to begin with. They're silabs based..which is nice...but they've got slow fets so they can't do damped light which is what really makes a big difference.
You can still recalibrate them with the usual methods. This is my 280pro with real blheli flashed to the AIO board's ESC's so the sounds will be a little different...but the basic technique is still the same. Recalibrating is highly likely to solve your issue with one ESC starting a little slower.
Now...the bad news. That's really not a very good RX to use on a quad. The reason is it doesn't have a failsafe mode that the flight controller can detect. It will just lock on what it was doing when you loose signal if you loose signal...and the FlySky protocol it uses is notorious for loosing signal easily. That's a great recipe for a flyaway. At a minimum make sure you've got your name and contact info on the quad before flying if you want any chance of getting more than one flight out of it. But ideally you should look at a better RX. If you're handy with a soldering iron you can make one from an A7105 module and an arduino pro mini both of which are available on ebay for a few bucks each. It will also be a PPM output RX so you'll only have to deal with one wire so it's simpler to wire up. That will let you configure it to drop signal completely which will allow the FC to go into failsafe and let your quad drop instead of flying away. Alternatively banggood sells an RX they call the "dasmikro" which has a working failsafe, though it's a bit trickier to use since it has to be setup to output an artifically low value on signal loss. And that means using your TX to output that signal then raising your minimum output on the TX and loosing a bit of resolution. It works, but it's not ideal...and the RX is very fiddly to deal with and has limited range due to a poor antenna.
A better option is to replace the RF module on your 9x with either a DSM(x) module or a FrSky module either of which will give you a far more reliable link and the ability to use serial RX's which can cut down on latency. The 9x is a great radio...but it's stock RF system is really marginal and in some cases downright dangerous. Having experienced a flyaway myself I can tell you first hand it's a horrible feeling watching your quad fly away as you stand there helpless to do anything about it. Thankfully I had my phone number on mine and the guy who found it a few days later called me and returned it. (My failure wasn't a flysky link issue...it was physical failure of the PPM wire on an OpenLRS setup combined with me not realizing I had lost my failsafe settings on the firmware...but it's still no fun!)
