I believe you can convert your bike to a trike. . .
You are correct. It's
fairly straightforward, not cheap. But I think the purpose built ones are nicer because they can be built with a lower BB/CoG and perhaps a bit more optimized rake/trail geometry.
They do make upright adult racing tricycles, but they are uncommon, and not for riding on sidewalks at 8 mph.
Well, nothing to prevent them from being ridden slowly on sidewalks
but yes, they can be so much more than that. Oh yes, so very much
more.