I usually use compressed air for every loose dirt, a bit WD 40 and a rag to clean the motor can, all other metal parts and the chain. Sometimes a toothbrush for the chain if I need to. After that I relube the chain and every now and than the wheel bearings.
For regular motor maintenance is just compressed air my way to go. When noises, unusual drag or vibration occur you can open them up clean thoroughly with compressed air and a toothbrush, testing the bearings by hand also is a good advice since one might need to get replaced.
But over all are brushless motors are easy to service nothing to special that could go wrong by design basicly. A bearing might go bad over time sure, it can also happen that a magnet gets lose or something is physically grinding or blocking the motor but basicly nothing else except maybe brut force like a crash or something.
Ah and BTW: Make sure you don’t put your motors on a workbench/place where you ragularly grind or saw metal (or clean carefully before you do that), otherwise the neodymium magnets pick up a bit of the flakes and may grind your motor to death rather quickly. It’s also kinda hard to clean that stuff from the motor even if you open it up without a stronger magnet on hand.