Could you provide a photo of the location, indicate the light and soil conditions and say where you are situated geographically? To offer useful suggestions we need a lot more information.

If the bush is needed to screen a view, I presume it should be evergreen?

My tree hydrangeas in Z5b are 6-8 feet tall and bloom for a long time.

Do your Hydrangeas attract butterflies? I’ve only ever grown mopheads, which attract nothing.

Most blooming bushes in zone 6 are not evergreen, so you wouldn't get full privacy year round. How tall of growth do you need for privacy?

zone is simply lowest forecast winter temp.. it tell us nothing more ...

z6 is almost half the US ... and its so broad.. that you have to tell us where you are.. so we can recommend appropriate plants ...

big city name is good enough ..

and a pic or 2 of the site would really help .. how tall do you need.. how much space do you have a ground level ... how big is your garden yard.. etc ...

ken

The largest flowering bush in the mountain top garden is a rhododendron that is about 25' tall and 15' around. For you I will recommend Viburnum rhytidophyllum,

Viburnum rhytidophyllum - Plant Finder (missouribotanicalgarden.org)

Hydrangea paniculata and H. quercifolia do attract pollinators in abundance. Both also self-sow readily which makes me question their invasive potential.

Also Magnolia stellata and Viburnum Chenaultii.

Screening is too subjective minus size requirements, so all I will offer is that my largest Joe Pye Weed 'Gateway' had literal swarms of butterflies all season. Serious quantities.

Ground to top of tallest bloom was eight feet and this is their first full season, put in last summer.

Judging by your exact words "tallest blooming bush hardy in zone 6" I'd say just get one enormous lilac and stand back for ten years.

https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=278791

https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=c230

https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?kempercode=g830

https://www.missouribotanicalgarden.org/PlantFinder/PlantFinderDetails.aspx?taxonid=281674