Wow. You make me look bad. Most folks with $1500 box store mowers won't spend more than $150-$175 for an annual service. I don't do all that you do. I need to spend 2 hours or less on an annual maint service. JD mowers are different. Those folks will pay more. The local JD dealers are $350-$400 for annual maint. I am around $250 for those. And i do change all filters and spark plugs on every annual maint. I throw away probably 100 good plugs a year.
Spend a bit more time on the services and stop throwing away all those plugs.
It is the throw away mentality that is destroying the planet .
Put a note on the bottom of your invoice stating that to help reduce pollution & global warming parts are now replaced as required rather than by scheduled service hours.
If necessay you can also tell your customers that you saved them from buying $ XYZ worth of parts that were not needed yet .
That will get the skinflints on side
The more you do the less time it takes as the initial greasing of the rear wheels can take 1/2 hour a wheel but the next time it will be less than 5 minutes because the greased wheel will slide right off .
While you can check he PTO oil seal with a cotton bud a visual pulley removed is better and again once greased the first time subsequent checks will have it falling off and you get to check the PTO clutch searing at the same time.
You can go one better and print out some nice looking check lists with everything you do on it and I mean everything like degrease dip stick cap & tube threads , Check engine breather , tyre pressure in & out for each wheel battery voltage engine off , engine on , engine on under load .
Then alternator output voltage ,at X rpm . Grease the seat slide rail and leave it in different position so the owners knows you have doe some thing with it .
Get a white paint pen and draw a line all the way round every belt & use this to run the belt fully around the pulleys, again takes a minute then on the check list note X no of burns & y no of cracks