To which sub-caste of “Kshatriya” did sage Vishwamitra belong to?

I've been looking for Vishwamitra's sub-caste to no avail. You are correct that he was born into the Kshatriya (ruling class); he later attained the brahma rishis, which is the highest form of rishis.

I will continue to look for the sub-caste.

Rajarshi Kaushika ( AKA Sage Vishvamitra) was the son of king Gadhi of Kanya Kubja dynasty and was a descendant of Amavasu son of Pururava of Chandravansha.