Metal wire behind lower front teeth.

A fixed lower retainer is not the solution for everyone.  Yes, it is possible to remove the fixed wire and replace with a removable retainer.

yes you can. You used Invisalign to get them straight. Why not continue with an aligner to keep them that way.  Invisalign makes a permanent retainer called a vivera retainer.  It is a little sturdier than your last tray but is designed to hold up longer.  And you will be eligle for reduced cost aligners if you have to re do the Invisalign due to relapse.  The bonded lingual is the most predictable retainer but it is hard to clean around can be irritating if not properly smoothed when placed.