Echinacea (coneflowers) are really full sun plants. They are most likely leaning because they are stretching towards more sun. I'd suggest you relocate them to a more suitable location and select some other type of perennial that is more appropriate to a morning sun/afternoon shade location.

Here is a link that might be useful: zone 5 part shade perennials

Years ago I had echinacea, and some leaned when in full flower, even though they were in full sun. I used those shorter round tomato cages that are not really strong enough for tomatoes, but they worked well with the echinacea and were not very noticeable. How much room do you have? Rudbeckia are a bright contrast and more stiffly upright, perhaps could support them? I've never tried that, but the flowers compliment each other.

I have Echinacea in my city and country gardens - in the city where there's been a ton of rain they got extremely tall (almost six feet) and were leaning all over so much I actually had to cut them back this year - in full sun sunrise to sunset, perhaps too much nutrient rich compost added to the soil. In the country gardens with less rain and no amendments (other than naturally fallen leaves etc) they stayed shorter and are straight with no leaning problems.

My favorite Echinacea partner in the country is a variety of Black Eyed Susan that stays about 12 - 18 inches high. I've had it for so long I don't remember the name, but I can say it spreads both by seed and runner if it's happy in its spot. This is NOT the Black Eyed Susan that gets really (two feet or so) tall, and that has wider leaves.

Putting something in front of the coneflower is a good idea because the foliage is not particularly attractive and it is not a very "structural" plant. I like to use a plant with good structure, like sedum autumn joy. That blooms the same time as the hydrangeas and coneflower so it would be a nice late summer display. Other choices would be peonies (depending on the height of your coneflower which comes in many varieties and heights), lady's mantle, hostas, daylily - all of which are easy care and tolerant of a variety of conditions. Have fun!!

Perhaps something orange and "fluffy" like one of the shorter kniphofia cultivars (red hot poker). The shape and color of the flowers and foliage would be a good contrast to the echinacea. The two shortest are 'Earliest of All' and 'Little Maid.' Both are 18-24" tall.

I've not grown these yet, but bought some seed this year and they are described as being drought tolerant with stiff stems.

I like to plant Liatris (sp?) because the colors are very similar. Of course it depends on the bloom time of your cone flower.