You may want to try a strap, even for a sitting posture. Your height might counsel against the footstool, but try some books or a coffee can under your left leg and see what it feels like. Don't worry about what anybody else does, find a way that is comfortable and allows all the mechanics to work. the footstool can hurt people, but it doesn't always. I've never had any trouble in over 50 years, but I still play standing with a strap for a nice change in ergonomics and some movement below, and I find it quite comfortable when performing jazz styles, prefer seated with footstool for classical. The lower curve fits into the left leg, the bout rests on the right, check out some you tubes, find Fred Hand's excellent older video to get started.
Lester Devoe Flamenco Negra
Lester Devoe Flamenco Blanca
Aparicio Flamenco Blanca with RMC pickup
Bartolex 7-string with RMC pickup
Giannini 7-string with Shadow pickup
Sal Pace 7-string archtop