I find rotating in a 5th tire a pain in the ..... Besides it throws off rotation IMO unless one is literally rotating before any noticeable difference in wear front to rear is present. Even still they wear different and putting fronts on the back rotating the right way will shape them back up and vice versa.
Spare tire if needs replaced, literally costs as much as a couple cases of beer buying off Craigslist. Same deal if you really want a matching rim. There are tons of sets and single take off wheels for sale cheap. A single factory rim of most any sort is relatively worthless and can be had cheaply. No need to buy new.
Just a couple suggestions.