We have 6 numbers to work with, so that’s what we need to use. Instead of 359 like on a 10-digit keyboard numeric pad, you can use 236 or 541 (upside down).
Look at your key and make patterns out of that; forget the normal keyboard and the 10-digit keypad.
Or better yet, create a random 10-digit base-6 number. To use it on OnlyKey, add 1 to each digit (because OnlyKey doesn’t have a 0 key). Start with memorizing 4 digits, and keep adding (and changing the actual PIN) until it’s a 10-digit (sort of base-6) PIN. This will maximize the security of your PIN code.