Need height increase cushion on my ergonomic chair

I purchased amazon's solimo ergonomic chair. Height ranges between 45-55cm. I has set the maximum height.

I like to sit in legs folded because my body loses heat quickly and finger tips of hands and feet get so cold during winters.

My table is a bit steeper, causing by shoulders, elbows painful as hell while typing. All the chairs are of the same range 45-55cm.

I purchased tailbone cushion in the hope of fixing it, although it raised height, my legs were not fully touching the ground, only thumb is touching. I placed a plastic stool, now my legs were at 90, but there's 5 inches gap between my knee and the edge of cushion. So, under thigh support is not full.

Looked at various cushions, either they are tailbone cushions which are countour shaped, or they are poor quality cushions.

I'm not really sure whether such height increase cushions exist to cater my folded legs posture.

Pls let me know if you had similar experience or had the knowledge of such cushions