This is a good start, no worries. If you have another Glock, go ahead and disassemble it, and look for every single tiny speck of rub wear. That's essentially your friciton points, but they won't necessarily show up on your cerakoted Glock for a while. Those would be the places you'd want to hit with your sandpaper - just keep in mind that you want it to be as smooth as possible, so try to finish with the finest grit you can. A heavier grit on cerakote (or steel) can actually add friction by adding friction-surface area by making the surface more 3D, with deeper ridges and valleys that grind against each other like stuck gears. You can do the initial resurfacing with a heavier grit, but definitely finish with as fine of a grit as you can.
With it sounding like consistent FTEs first and foremost, and occasional failures to chamber, that indicates the excess friction is occurring on that backstroke. It sounds like you already did the rails, but maybe hit those again too. That said, my firmest suspicion is that the locking lugs may be binding a bit on the cerakote, especially if it's a bit on the softer side.
Glocks use the barrel hood as the "locking lugs". The front of the hood locks up against the font of the ejection port. The back of the barrel hood locks up into that notch at the back of the ejection port, just above the firing pin hole. There's a pretty decent chance that some visually imperceptible excess is in those corners at the back of the ejection port where that hood locks up against it. Anywhere those locking lugs come into contact with the slide would be a good place to take down a bit and smooth out.