You are talking about 3 different types of maintenance: boat, motor, trailer.
Boat maintenance-- I do myself including electronics, trolling motor, batteries, etc. except if I see a spider crack then I go to the glass shop and have it fixed
Trailer maintenance-- I check my brakes, fluid, lights, tires, bearings, etc on a regular basis. My trailer is 7 years old with the original bearings in but I keep them maintained with marine grease. I have taken them out once, inspected them, put in new seals, and repacked them.
Motor maintenance-- I pull the prop and check for mono behind it a couple of times each year. I change the lower unit lube every 6 months. I change plugs every 100 hours. I have the impellor changed every 3 years. Once a year I have a moonlighting tech come by the house and give everything a good looking over.