I've gone through the same magical mystery tour with the 12.2. Slow charging, screen flickering at 25-30% and also ~60%. Even went so far as to replace the port, battery and re-soldering of the 6pin connector between the port and battery cable. All see well for quite a while then the dreaded issue returned. This time I monitered the actual cable plug with the port. Unlike USB C that seem not to have the problem the USB 3 port plugs gen loose after a month or two. Plugging in a fresh cable, now, returns the charging to normal status. I've gone so far as to experiment with ALL cables including factory (of course). All of them have the same problem of loosening connection over time. The more rugged brands/types do tend to stay tighter longer. The second issue with the cables is the bending at the cable and USB 3 plug. Too much ad the same problem can manifest.
This is a hassle but a solvable charging issue.
This is a hassle but a solvable charging issue.