You see this is why I don't get into these debates - you can never prove anything either way.
Yes Qaraar suggests it is sedentary, fixed, and does not move. But Muslims today will of course claim that means it is just stable/steady in the sense that it is not shaking about and does not mean the earth is not moving. It's possible it can have both meanings - though it's odd that Allah did not clarify things a bit better seeing as people thought the earth didn't move - and seeing that man would later discover the Earth's movements.
Anyway this is from Tafseer Ibn Kathir:
أمن جعل الأرض قرارا أي قارة ساكنة ثابتة لا تميد ولا تتحرك بأهلها ولا ترجف بهم فإنها لو كانت كذلك لما طاب عليها العيش والحياة
""The one who made the Earth fixed" means sedentary, still, fixed, not swaying and not moving it's inhabitants and not shaking them, for if it did that, then living and life would not be able to flourish upon it."