I'm a bit further south than you. I seemed to lose 1-2 of my 8-18 delphiniums every year. Been covering with straw the past couple of years and only lost one to the foot of a helpful grandson this year and the remainder are subjectively more robust than when they would come back w/o winter protection. I'd keep replanting anyhow since its a favorite.
Will

Perhaps good drainage is the key. I have some long-lived ones in a raised bed that gets good snow cover in the winter. I believe they were Magic Fountains. The light blue has been my survivor.

Mine seem to get decimated by insects, usually ants, but I am trying again this year.

A good way to get rid of ants is to mix about 1:1 ratio of sugar and borax and put a little in small bottles and lay them on the side in the areas frequented by ants. The Target pharmacy bottles work great, because they are flat on the side. The sugar helps to attract them to eat the borax which kills them.

I have been growing delphs for 12 years in the Twin Cities. Mine grow like crazy with little care. In both of my homes I planted them where they got morning sun but were shaded in the afternoon. I also grow only the New Zealand Miliniums which are supposed to be a superior variety. Most were started from seed or just reseeded from mature plants. The first time i grew them I bought some large plugs. I'll try to attach some pics.
Anne