I do not think you will be able to keep an avocado tree alive in your zone without significant winter protection (a cold frame/temporary greenhouse each winter, probably with a heat source for cold snaps).
If you did want to try zone pushing avocados (and I’m in no position to discourage that, it’s my obsession!), you will need to start by finding cold-hardy seedlings for rootstock. Grocery store “Hass” seedlings have not yet successfully survived in my 8b yard, above or below ground, so I don’t think they make suitable rootstock for zone pushing of avocados.
That being said, one thing you could start with is simply planting all the seedlings you have outdoors now. If any of them sprout from their roots next spring, they could be suitable for grafting low (below ground level) with a hardy variety, which will still need significant protection in your climate.
If none of them sprout in spring, you’ll know you need hardier rootstock.
Here’s an example of a “hardy” seedling (Mexicola Grande is the parent), starting its third growing season here in zone 8b, you can see the remains of the first and second season to the left:
