Hi Tan, my boys tried out a harness for quite a while when young and always (as I'm sure you'd do) under supervision. Max never really took to it, he'd go back indoors quite quickly, but Swampy used to purr at the sight of it as it meant outdoors. Some brands look a whole lot comfier than others though I can't recommend a make. I also used it on one of those long, extender doggy leads - which added to muscle tone for an indoor cat too (the hunk of the neighbourhood he was!).
The other day someone in the street came past with their cat on a harness going for a walk - cat had long lead and walked along the low wall quite happily. Although what they do when s/he sees a dog, I've no idea! I got some funny looks from neighbours back when I did this (small London garden in the 80s) as they all had outdoor cats too, but I didn't give a
as my cats were having fun and were safe.
There was some minor disappointment when the door shut afterwards, but not for long as long as other 'entertainment' is provided
(catnip mostly).