A chair is fine. A cushion is fine. What matters is that your spine is upright without strain, your shoulders drop, and your hands can rest.
The three-point check
1. Sit bones grounded. 2. Spine long, as if a thread lifts the crown of your head. 3. Chin tucked slightly, so the back of the neck feels open.
If something aches after a minute, adjust. There is no virtue in suffering the posture.