Few more pictures of nearby areas that also need to be filled in.

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Prunus laurocerasus / English or Cherry Laurel
full sun to part shade - even tolelrates deep shade .
Cultivar Otto Luken is smaller - 4x6 and likes deep shade, there are other cultvars that can grow to 15 ft.
Water will in the beginning, ( like any other plant) but esp since they will be planted in the root zone of the big spruce.
Zone 6 -

Here is a link that might be useful: Garden Design Ideas

Cephlotaxus or Taxus, depending on how bad deer are in your area.

do NOT even contemplate in between them ... too much water competition ...

first pic.. come at least 8 to 10 feet toward the camera ... and you will succeed much easier.. and faster ...

do not get sucked into planting everything along the lot line.. [i am presuming you are trying to hide the neighbors house behind] ...

the fact that the myrtle .... is alive.. but not aggressive.. alerts me to such ... it wont grow between and way under the tree.. only thriving near the edge.. where there is more light and less root competition ...

are there power lines running thru those trees???

ken

Dragon Lady Holly or English or Cherry Laurels.....maybe Maxi or even Roseum Elegans rhodos also.
I have some small blue spruce only 4/5/6 ft tall and losing lower limbs in the future is a major problem as they need a ton of light. In anticipation of losing lower limbs in that future I have planted Blus Star Cypress which have a similar color and are evergreen and kind of blend in. In your case with mature trees I dont think that recipe would work.