Ahsan Allahu Ilaykum Sheikh, if someone from the West wants to wear a Thawb, but his parents are against it while adhering to Bid'ah and not even allowing to shorten pants above the ankle. Would it be permissible to wear a Thawb and shorten it after he gets married ?

Wearing below the ankle is not allowed, so shorten the trousers and speak to your parents with wisdom, kindness, and make dua for them. But the thawb itself is not required, and Shaykh Ibn Uthaymin said the Sunnah is to wear the permissible dress of your people, so there is no need to fight your parents over it.