Except for the ones with fragrant flowers viburnums usually have no apparent aroma or they actually stink ("aromatic" in reference to plants tends to mean the foliage has an agreeable aroma).

Try clethra or summer sweet flowers only. Look up a plant called sweet fern the foliage smells really good

Viburnum carlesii (Koreanspice viburnum) 'Aurora' and 'Diana' are both a bit smaller than the typical Viburnum carlesii, in width and also in height, so might fit. Pink buds and white, nicely fragrant flowers. Neither should exceed 8' in height. I really like the scent of this plant, and the foliage turns a nice mahogany, with a few leaves of gold an orange in the fall.

Some of the larger Fothergillas might meet most of your requirements, but unfortunately need organic, somewhat acid soils, so I guess wouldn't work.

Betty Corning clematis is scented and tends to grow large, but might work with the right shape obelisk or trellis since vines will grow to the trellis height and then flop down over themselves.

It might be helpful to see a picture (I'm a visual person), but you might think about adding some contrast in form to your bed. If you haven't already purchased the barberry shrubs, you could choose something else for accents to get more contrast. The growth habit (mound-like) is similar to the knock outs, and the foliage color of the new growth on the roses may be similar as well. My red knock out foliage comes in red, similar to the crimson pygmy barberries, though maybe that's not the case for the pink knockouts? I don't have any pink ones, and it's hard to tell from pictures on the web (looks like the foliage isn't as dark as the red KOs anyway), so I could be wrong. In any case, have you considered adding any dwarf conifers for accents and winter interest? Or ornamental grasses if you like those? The iris will be a nice contrast to the mounds.

I think you could do a viburnum in the corner, maybe something that stays smaller, though I wouldn't go for one that is mound-shaped (like dwarf American Cranberry), but something that has a shape that gives you some contrast to what you've already got in the bed. Maybe a Korean Spice Viburnum, which is supposed to stay 4-6'. You could also plant a larger viburnum (like Alleghany or Mohican) but prune it to a small tree so it doesn't crowd out whatever is around it. Just a few thoughts.