Early retirement.

[Razgruff]

Coincidently I was just thinking about grass cutting, all I would need is a petrol mower...

And somewhere to dispose of cuttings, most people want that stuff taken away,It also will invariably involve weeding and lawn maintenance.

A trailer will become essential along with strimmers, hedge trimming will undoubtedly become something asked for as well. people that get their lawn mown generally want all the garden jobs they can't be bothered to do done for them.

if you have back issues I wouldn't attempt lawn mowing lots of lifting , pushing mowers up gradients even if mower has drive you will still need to put effort in on inclines.

Ride on mowers unless professional models with wreck you back also.
Have you thought about craft type work, upcycling stuff, old bike parts into clocks tables etc,