You can do ugc bridge course in mathematics to pursue a B.Tech,but it is limited to certain university and bridge course from NWAC board, though valid with an AIU equivalence certificate is less acceptable in India and might create problem in government roles.
so the first option is safer with some drawbacks.
Additionally,for a math bridge course recognized by IITs and other B.Tech colleges, you should use the NPTEL platform on SWAYAM .
Good luck!!
Regular exam updates, QnA, Predictors, College Applications & E-books now on your Mobile