An offset flange is a pain from the beginning, and compounding those issues with one that fit inside of a 3" pipe, effectively making it a 2.5" pipe would pretty much guarantee clogging on a regular basis. Sealing a toilet to an offset, oblong flange is somewhat of an art, but doable. You often need more than one wax ring, cut them, splice enough together to go around the perimeter, and set the toilet. A little tricky, but not horrible. Plumber's putty tends to dry out, shrink, and crack, not the best long-term solution there.
Your best, long-term solution would be to crack concrete and move things to use a standard 12" rough-in with a stock flange. Barring that, a straight flange to a 10" can work, but you're limited on the toilets that you can use, and your total costs may end up higher. Anything other than 'standard' tends to cost more.