I'm sorry.. I wish I had something pretty and shade tolerant and zone appropriate.. but all I can think (if I am going to be kind) is Mulch.
But If I'm going to be the pusher man... you asked for it! Lol
You have a couple issues
Northern zone
Mature trees have dense roots and not a lot can compete
Shade
So, little soil, little sun, hard winters, short summers, yeah. Mulch.
You clearly did a ton of work. I've been "reclaiming" yard and I know how much work you did. The idea there is an inexpensive fast growing shade loving maintenance free .... pipe dream.
What I would do is landscape cloth and commit to long term project. rhododendron near/ in between trees and hortensia (hydrangeas) just a little bit further front. They will tolerate shade, harsh winters, live forever and grow bigger every year. Each year will be less work than the last.. in 5 years you'll be in really good shape for30 more.
Do research on rooting hydrangea, if you have friends w this bush. They grow very dense, keep the weeds out and are stunning from mid summer to early fall. They are pretty easy to root. If you take cuttings early, by the end of this summer you could be golden..
If there was something pretty and neat and inexpensive, i think it would be living there already and you wouldn't have broken your back to get it looking as good as you have.
Don't forget the landscape cloth and mulch, so weeds don't pop up the minute you turn your back,
