Whew... I'm so glad you're here, keeping the auto parts people in $$$. Really, the maintenance you're doing is great for safety. I do not fault you for taking care of your family.
I even think changing spark plugs sooner than 100k is good; maybe 50k. Though, annual brake pads, way too often. Waste of money and time... No wonder why you don't like cars. Yes, you did good changing the pulleys... Though unless making noise, replacement might not have been needed.
You selling auto parts, makes you a experienced mechanic? That's like being an pharmacist or maybe an EMT making them doctor.
You are right, engineers have a great job. How the are sitting at home with their $$, laughing at us slobs. My kids would complain about their school teachers...'they hate me dad.' Yup, they became teachers just because they don't like kids. I'm not defending engineers and I have too cursed their profession. Though I again, challenge to design something better.
Just maybe, changing serviceable parts is partial the problem.
It doesn't matter to me what you do. My initial point was to help in not wasting time and money replacing serviceable parts. Really, it's your money and time. As well as, you are keeping the road safe.
Though, my truck is due for a wash and wax.. maybe I need to swap the engine. Ya, know it's over 100k in miles.