Nothing new under the sun, I'm afraid

Lots of people done this sort of thing already. My take on it is that if you want lots of 'one off' type jobs, where you do them and that's the end of it, then there are probably plenty about, but to my mind a basic quality of window cleaning is the repeat business.
'One offs' are fine when you haven't got anything else to do, but it's far better to have a full quota of window cleaning work so each and every day you know you have X amount of work waiting for you and you don't need to go looking for any more.
You can only do so much - use your time as effectively as possible - build your round and concentrate on refining it so it pays you the maximum income for minimum effort.
'One offs' are always a bit of a gamble - you may make a fortune, you may waste a day doing what turns out to be an impossible job - and what about the window cleaning work you should be doing while you're doing the one off? You can't do both at the same time and if you have spare time for doing one offs, what happens when you run out of them? You then have unproductive time on your hands.
Give me the tried and proven guaranteed earner every time.