I’d fly from DC. We drove and it was miserable - you hit every major city along the east coast.
I loved Montreal in the summer. Street fests, good food, nice parks, easy metro, great walks. We stayed at a Marriott with a nice indoor pool/sauna. It was warm and very humid while we were there.
You can easily take a train to Quebec City, which was charming but very touristy. In QC we stayed in a little airbnb in the old town, but now I know it’s not necessary to be right in the old section.