I have an Ender 3V2 running a custom version of Jyers Marlin, and using a CR Touch as a Z-stop, and with a heated glass print bed. This is my first printer, and it mostly prints fine except when printing near the edges where some imperfections with bed leveling mess with bed adhesion and have definitely ruined a couple of my prints. The main issue is that the first layer is very uneven, where in some places filament it basically smeared onto the bed whereas in other places the filament is barely touching the bed. What's stranger still is that generally speaking, the infill seems to be more consistent in terms of z-height on the first layer, whereas the walls have much more variation. I've included some photos to illustrate the issue:
This is the entire leveling test I printed (it only tests the edges because the center is fine). This was run without UBL - I verified this as the Z axis motor didn't move during the print (it's only 1 layer).
This is what has been causing me the most issues. In the top right corner of the print, the wall filament basically disappears for about 1-2 inches because the nozzle is too close to the bed. This is pretty close to the top right leveling screw as well. You can also see that only the walls are affected, as the infill is pretty even across the same area.
Kind of the opposite issue in the bottom right compared to the top right, where the walls are printed too high off of the bed and start to string from poor adhesion. Once again, the infill seems to be ok (except in the corner).
Things I've tried:
Leveling with a feeler gauge. Incidentally, I always had issues with the top left and right leveling screws, where the top left screw would always be too high up and would be hard to adjust because the screw would be too tight, and vice versa on the top right where the bed would always be too low and the knob would almost fall off by the time the top right was leveled high enough.
Tightening/loosening belts. Doesn't help much, but I'm also not sure how tight the belts should be anyways.
Enabling ABL/UBL with G29/M420 S1. It just... doesn't seem to work, the leveling print will come out the exact same each time no matter if ABL is activated or not. I guess this probably means it's not an ABL issue, or that if it's an issue with motors etc. it affects the sensor while it's leveling. That's the only explanation I can think of.
Readjusting the frame so that everything is as close to square as possible. There is no wobble so I'm 99% sure that it's right, but I don't have access to one of those square rulers.
The Z screw does not move/wobble when the motor moves, I re-tightened the coupler for the screw and the motor.
I have not adjusted the X axis bar that holds the carriage and moves up and down, because the nozzle seems to be a little higher the further away from the side with the Z screw it is. From other help requests I've read online, it seems that usually issues with the X axis will result in the nozzle moving closer to the bed as it gets further away from the Z screw, which is not the case here. But would it be worth it to adjust it anyways?
Given that this is my first 3D printer it's really kind of put a damper on the experience for me as even though 75% of the things I print are small enough to avoid the problem area, I want to be able to use all of the print bed without issue. Any help, troubleshooting, or advice would be greatly appreciated!
Edit 1: Took off the X/Z carriages and found two abnormalities - not sure if these are causing the issue. The nozzle carriage has a bit of space between the bottom of the X-axis profile and the roller. It doesn't really wiggle during printing, but it could if I touched it; right now, the two rollers on top sit nicely on the profile. Also, these rollers all have a slightly discoloured ring where a little bit of material has been shaved off of the roller. The Z-axis rollers on the side opposite the lead screw are the opposite case, where the rollers have to be forced onto the structure since it's so tight.
