I don't agree with the approach of getting used to some support, chair, footstool or whatever. It must be the opposite. You first find the best position for the guitar holding it in the air, maybe someone else should help with this. Then you adjust an accessory, footstool or whatever to hold the guitar in that position, trying to achieve with good overal ergonomic for the whole body.
My previous teacher had an accessory, a kind of pedestral similar to Aguado's tripod, to hold the guitar completly without needing to hold it or touch it to your body. He puts the guitar high with this and ask the student to play with it standing up. Then he researchs a nice position for the guitar with this support while the student is playing standing up. With this approach both guitar and body are very free and easy to move around. After finding a good free posture/position, he works translating it into sitting with a regular accessory.
When I first changed to an accessory, from the footstool, I found one and an adjustment that could keep the exact same position of the guitar relative to body, hands etc., when compared to the footstool solution.
It doesn't mean position/posture should be treated as a fixed thing. You ceratinly may do small changes, we are always changing as we make progress. Maybe the change with the accessory showed you that your previous posture treatment was not so good and you should research more.