Evergreens like our clay soil. Crabapples as well.

That mountain ash is a goner. They require better drainage than the average tree.

My advice to you, and you're not going to like it, is to amend the area so that you have 12 inches of soil. Dig out the spireas and the rhod(!) and replant them.

I have clay soil. Trees and shrubs that grow very well in it are Lilac, Ninebark, Willow (there are smaller willows), anything in the Prunus family (like a Mayday tree, or a Shubert Chokecherry), High Bush Cranberry, Dogwoods, Barberry.

Stay away from roses

The thing about clay soil is it is very rich and holds moisture well. When it dries, tho, it's like a brick. It also hinders the uptake of iron which makes a plant green so that's why the plant looks yellow (or chlorotic). Some are more susceptible.

I find it much easier to plant things that like my soil instead of attempting to amend the soil. You can never really amend it enough to make some plants happy - perhaps for the first year or three but deep rooted plants (like shrubs) will quickly bypass that 12" of lovely soil and get down into the clay anyway. (And that's what happened to my roses). And when you plant things that like your soil there is no battle and the plants really grow well and look healthy. Plants that like clay soil are generally plants that like alkaline soil with a higher PH.

You may have a bunch of 'fill' in your yard - that stuff the builders dug out of the soil for your basement then they spread it over the land again when they backfill and level. That fill may only be a few inches deep and then there might be good soil under there again. You'll never know unless you dig a hole and see what levels of soil you go through.

I have Spireas - some are great and some are a bit chlorotic. But I think your Rhododendon would prefer an Acidic soil, or that which has a higher PH. You could try an acidic fertilizer (Miracle Grow makes one) which may green it up.

He still needs to do something about his topsoil. He only has 5 inches. That makes it very difficult to grow anything at all. It's not really a matter of amending - he needs to add soil.

I have yellow clay throughout most of my yard. I agree with the previous suggestion for lilac, ninebark, shubert (if you aren't worried about black gall) and viburnum. All are doing well in my yard.

I don't think you should give up on the spirea, it may be just a different variety you need. I have a large gold mound spirea that was here when I bought the house. It sits in totally unamended clay and is lovely.

Also, nanking cherry seems to do well in clay. I planted two about 6' apart--one is in a nice mix of soil and the other is in pure clay with about 4" of top soil (the grass). The clay was so hard I had to chip it away to create the hole. I dug a 3 foot wide, 2 foot deep hole and amended the soil that backfilled the hole as best as could. After two years this nanking cherry is twice the size of its companion (much to my surprise!).

Spruces and larch will do well in clay. Most pines prefer a sandier well drained soil. My currents and saskatoons like the heavy soil. Lodgepole pine tolerates it where there is a slope for drainage.

Our garden was largely clay when we moved in. Took me a while to realize that the clay held on to moisture so well that I really didn't need to water it. So I use tomatoes as water guages. When they go a bit limp, I water the garden.

The other thing I've been doing is sneaking off to the city with the truck and trailer, and creeping down the alley ways in fall. Bring back a couple loads of bags of leaves. Spread out 6" thick on the garden, and rototill in. Been doing that now for several years, and the soil is getting better. Admittedly I've gotten some new weeds You could do this with a green manure crop such as vetch too. this way.