Then don't! If it's autonomous it can be out there all the time rather than once a week/whatever, no spinning blade needed. Plan B? Buy a goat.
Nah. What we really want and need is a robot that while it mows the lawn, turns it into: its entire energy source, its entire pool of building blocks for daily (nightly?) self-repairs, for reproduction, for producing oh maybe fluffy warm wool and delicious milk/cheese/butter as a side product, and fertilizes the land from the occasional .. discharge resulting from perpetual energy production-consumption and internal cleanups/repairs, finally as it still does wear down as all physical assemblages are wont to with time, it leaves behind highly durable inputs for sturdy clothing and stylish home decoration (horns, hides, leather etc)!
All from a friggin lawn.
Roboruminant 2.0 baby. Need to reinvent mammalian evolution before we can really reinvent the wheel!
I'd call that safe enough to operate unsupervised in an enclosed yard.
It's probably cheaper to hire a landscape maintenance company, or a neighborhood teenager.