If you can't grow acid loving shrubs, you ain't growing clethra. End of story.

Lavender is a distinct possibility, as of course is boxwood. Both will require a serious pruning once a year. I've like germander when I've seen it, but have no personal experience with it. Pennisetum or another smallish ornamental grass will work.

In general, I think the size is going to be difficult to achieve with shrubs.

How wide are you expecting the plants to be? Even shrubs that have been bred to be short can be wide, especially in bloom.

Not to sound stubborn, but clethra does grow here, it seems to tolerate our clay soil somewhat. I was referring to things like rhododendrons, azaleas, pieris, and such. I like the lavender idea, I have several around my yard and they only require a once a year shearing. I could go for one of the larger varieties even.

Ideally the shrubs should not exceed 3 feet in width...

Just a question, but do you prune off the Spirea before it leafs out in spring? I shorten mine to about 10 inches, just as the leaf buds appear.

The benefits are lush new growth, puts the bloom time back a few weeks, so everything looks nice at a different time.
Mine are Limemound and Goldmound, so the colors are quite different, contrast well. Other varieties might give you different leaf colors as well.

The short, Japanese Spirea, bloom on new growth, so your shearing them short, should help increase the bloom quantity too. My flowers last a couple weeks, as they gradually all get opened.

I love the little Spirea, they are always colorful, RELIABLE, no work except that yearly cutting back.

So an idea for you to consider. Change out the bushes if you only have plain green. Doing an every other variety, with two very different colored leaves, could be quite striking.

I usually tidy the hedge up a bit before fall so it looks decent in the winter, as decent as a bare spirea hedge can look... I like your idea of different colored spirea... I have always liked Goldflame spirea, right up to the point when it flowers... hate the pink flower color against the golden warm red leave colors...I'll consider it, thanks Goodhors.

some other suggestions that might work for you: Russian sage, caryopteris - there are several varieties with yellow or variegated foliage, leptodermis oblonga-very twiggy so has good winter presence, dwarf red twig dogwood, Lemon Queen -a yellow-leafed spirea that I think is superior to Gold Mound

Spirea Double Play Artist has deep purple/reddish new growth followed by deep fushia flowers. After the flowers fade I cut them off and shear the shrub slightly which triggers more new colorful growth. I am very impressed with this one and the size is right for you.