Have you tried any of the following apple root stocks, they are all supposed to be very cold hardy. And Malus root stocks which they are, are said to be good pear root stocks
- M.7 (EMLA 7) “produces trees roughly 50% to 65% of the size of a standard seedling tree.“
- MM.111 “produces a tree that is about 80% to 90% of a standard (seedling)”