With stable firmwares, you don't overclock GPU with the core_freq keyword. Only experimental/unstable firmwares do that. Instead of it, you can use hdmi_enable_4kp60=1 to overclock from 500 to 550MHz (Videocore GPU). You lose composite video output though, because it reguires cca 360MHz core clock.Here are my OC settings for 2140, pretty much the same as you suggest:
(and no, my warranty didn't void, if anyone is curious)
Keep up the good work!
Just an addendum, there are two types of crashes. One is a freeze, i.e. it stops responding. The other one is reboot - and that's caused by overload of its power management chip, which can't supply enough current at the preset voltage AND temperature. In other words, if it overheats, its maximum current output drops and when the BCM chip needs more, it reboots. The voltage at the 1V rail drops for a short while.I have two RPi4 - 4Gbs. The first I got to 2147, but after about an hour it would finally crash.