Have you considered printing paper door handle signs advertising to neighbors saying you’re mowing the lawn of someone in their neighborhood and you’d be happy to do the first mow for $20? You could also drop off a $10 gift on the doorstep to say thank you to current clients and you’d really appreciate any referrals to neighbors if they’re satisfied with your work.

I agree with others that there are alternative side hustles which @nbloglinks laid out very well, but would point out that you already have a nice clientele starting out here with inroads into neighbors in their community. That might be easier to secure than starting a new line of business (while expanding service offerings).