The must-fix-everything part of me might suggest getting them to contract the work out to you but your price will need to be a lot higher than your salary to co

The must-fix-everything part of me might suggest getting them to contract the work out to you but your price will need to be a lot higher than your salary to cover insurance, benefits you currently get, cover for when you want to holiday, etc etc etc. But some roles have to have built in slack to cover emergent situations. In your example, to be available in-hours to do clean up a spillage or unblock a drain. Something that would impact other workers. Actually filling your timetable with work would diminish that slack-insurance. There's no easy answer for working less in that situation, especially if you're already the only janitor on site at a time.