Gents, respectfully, the optic and slide cuts are non-issues here. It's understandable to suspect them, but let me explain why it's not likely:
The milled out portions of the slide likely exceed the weight of the optic, so it balances out. That's a lot of steel removed, and if anything it would increase the slide's reward velocity. People have been mounting red-dots to these handguns for roughly a decade with and without slide cuts, and it just doesn't cause any meaningful change in slide velocities impacting reliability.
When slide milling is a problem, it's generally because it opens up the gun's friction surfaces to friction contaminants (ie, sand, dust, EDC lint, etc). Or, when it's taken to extremes, it can take so much mass out of the slide that it may have trouble retaining enough energy and momentum during the forward stroke to overcome the resistance in stripping a round off the mag and then chamber effectively.
What the OP's described is mostly failure to eject, which means insufficient energy/velocity on the backstroke. Most of the additional FTFs he mentions are likely to be the result of insufficient forward energy, because not enough was applied to the recoil spring to begin with during the backstroke. You generally see this type of malfunction (FTEs) when there is either too little energy from the shot (ie, light ammo, heavily compensated race pistols, etc), or too much resistance from friction or springs. If he's shooting normal factory ammo, with his factory recoil spring assembly, then there's an excess of friction somewhere in the gun.
He also says he's lubed the gun and it still has these problems, and has repeatedly checked to ensure the gun was reassembled properly. Those would definitely be the first couple of standard things to double-check, so he sounds good-to-go there. All combined, the most likely culprit here is the cerakote job. It's easy to apply a little in excess, and if it's hanging to dry from the front, a little excess can flow right into the places where the locking lug surfaces meet up with the barrel hood. It certainly could be something else causing the malfunctions, but in one way or another that slide is just not coming back with sufficient velocity.