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I had pretty much the same problem on my Ender 5 except I had no issues with firmware, Creality had it for the upgrade to a 4.2.7 silent motherboard. The BLtouch was a nightmare, I could level the bed manually with a dial gauge, bed was perfectly flat with no warping at all, then run the BLtouch bed leveling and my print's first layer would be terrible, thin in spots nearly scraping the bed, thick in others and not sticking. I gave up after a week of frustration and having to turn off bed leveling every time I needed to print.
The BLtouch cable can be at issue if it's routed anywhere near a step motor or the hotend. Try turning the BLtouch so that the connector is away from the hotend and shielding the cable or replacing the flat cable with a twisted one. It's worked for some.
Did you use the adapter board? I've heard it can make a difference as well...
I returned the BLtouch, bought silicone spacers to replace the leveling springs and now the bed holds level really well. Just crank down on the pads so there's good resistance and they grip pretty good, not losing position to vibratio/heat like the springs can. I'm thinking about getting the solid aluminum spacers to just lock the level in permanently, just haven't decided yet.
I don't know why I couldn't get the BLtouch to work properly, it was like it sent bad information and read the bed wrong. I returned it before I found out about the noise issue with the cable so that could have been the problem, I upgraded to direct drive so the cable ran right by the extruder stepper. I've heard that the EZABL works better but I don't have any experience with it.
Good Luck!