Expungement is a nebulous subject. In my opinion, you’re fine. Get a full criminal records check (state you were arrested in, and Fed). See what comes up.
There are issues though.
Cities, counties, states, and regions do not necessarily share data AND/OR have their own “special” database system that is not part of NLETS. That information won’t be shared with Canada via an NLETS query; only NLETS data will be shared. I would bet $1M that the Canadians wouldn’t know you had a DUI conviction.
I’m not a Canada Border Services officer but I am a (“retired”) immigration/ customs officer for a nearby nation (and I’m also Canadian lol). In my experience, even if a foreigner admitted to a crime, an admission of guilt of a historic conviction is not substantial enough to permanently preclude their admission. Evidence would be necessary to substantiate whether or not they are definitively inadmissible. The burden of proof is always on the alien; it’s not the governments job to prove you’re admissible. IIRC, I would let them continue on their travel but tell them to gather the documents prior to their next admission. If they were a real sh*tbag I would boot them back to Canada and make it difficult. I digress.
I would be honest with your application for employment; more so if they **specifically** ask for expunged convictions (very common today). You could consult with your attorney but again, expungements are quite nebulous.