The first thing you should do is redo the bed line. Those tight, artificial looking curves are not attractive and will make lawn mowing a pain. And unless there's a gate in the fence at the far end, the side bed should attach to the one along the back of the fence.

Choice of plants will involve knowing sun/shade details.

I agree about making the curves more generous but not necessarily about joining the side bed to the back bed in the corner. Once the beds are full, especially if there is something evergreen and vertical at the turn, that could look like a path leading somewhere. In a small garden the sense that there is something round a corner that you could explore can make it seem bigger. A seat, large pot or other feature could be placed there or even a false gate or mirror on the fence. Such things need to be done carefully to avoid being twee. The other thing I would suggest is repeating plants, not having one of each so the bed has coherence and doesn't come over as a collection of unrelated blobs.

I tend to put the bed line in after the plants so that any gentle waves make sense as far as what is planted there. You want the attention on the plants rather than the bed edge, so let the plant design guide the edge. If you can spare another foot or two, it will give you extra room to add another layer of smaller plants so that there is more interest in the bed. Like Floral, I would suggest at least some repetition, though not all of the plants need to repeat. Perhaps choose one strong looking shrub and one perennial to repeat in 3 chunks, and the rest can be more random. I used a 3 varieties of evergreens, and 3 or 4 types of perennials in a larger mixed shrub bed to do this, but the rest is random.

Quickfire is a great choice, one of my very favorite plants in my garden due to long bloom, lovely flower panicle shape, and sturdy branching. It will need the entire 7' width to grow since it's large (plus enough room to reach around the back to do any needed maintenance), but it is so worth it. You could also plant a smaller Hydrangea paniculata in another spot in the bed to provide some echo without it being an identical plant. One of the things love about QF is that it makes a great trellis for clematis. My favorite combination is with a loosely growing clematis such as Little Bas, though there are many other clematis that I grow into shrubs such as Guernsey Cream and some of the bell-shaped clematis. Here's QF with Little Bas.

I use perennials and shorter shrubs in front of my larger shrubs and have clematis both on trellises and growing into stiffer-branches shrubs such as QF and lilacs. Think about what your goals are - do you want berries for the birds or shelter for nesting? If so, think about some of the evergreen or deciduous hollies or viburnums or other relatively dense berrying bushes (and don't forget the males if needed.) Do you want winter interest? Look at plants with interesting bark such as red-twigged dogwoods and evergreens (broad-leafed and conifers.) Do you have a height preference? Do you want lots of flowers? Look for plants with long bloom time. Is fall foliage color important? Also, don't forget to think about foliage color and texture so you have some interest and variety when things aren't in bloom.

You haven't given location other than zone, so be sure to know your soil (get a soil test if you don't know) so that you plant things that will be happy there. If you give us more info on location and/or soil, you will probably get more suggestions since MO has fairly different growing conditions than CT, though both have the same zone.