Im a revert and my parents dont know. I plan to tell them when im older and no longer dependant on them. If they force me to do a ritual that is completely shirk/kufr and I have no way out of it, in my heart I only believe in Allah, then I repent afterwards, would that still count as major shirk?

Real coercion is an immediate threat from someone able to carry it out that you can neither repel nor escape, and Allah Jalla wa ʿAlā (the Majestic and Exalted) excepted only that, the meaning of which is: except one who is coerced while his heart is content with faith (Surah an-Nahl, ayah 106). Fear of their anger or of embarrassment is not coercion, and where you are able to avoid the situation or leave it, that is what is required of you.