Grasses definitely make it prairie and not cottagey but you can include some flowering plants for variation in texture and colour.
Try adding in some verbena bonariensis which will have similar height to those grasses and light airy stems with small purple flowers. They look great mixed with grasses.
Another vote for Karl Foerster. It will stand from May until the annual chop in Feb/March, and will stay bolt upright barring some sort of calamity like a stray football or dog. There's a period when the flowerheads are quite fluffy and can bow slightly in the rain, but they soon form strictly vertical seedheads. I like it on its own but you could mix in some 'floaty' plants as Obelixx suggests.
"What is hateful to you, do not do to your neighbour".
I think grasses blend perfectly well with cottage garden plants and don’t have to look ‘prairie’ - I have recently added some grasses to a border with shrub roses, verbena bonariensis, gaura, hardy geraniums, catmint, penstemons etc, so fairly cottagy, and they fit very well. But that’s irrelevant really, it’s more a case of whether you think they will fit in your garden style and do the job.
Is it possible to extend the planting area out on one side? That would give you more scope to blend it in and make it look as if it’s a curvy design feature rather than an obvious disguise of a circular object that’s landed like a spaceship in your garden!
Another option would be some sort of decorative trellis curved around the legs that you may be able to grow some evergreen climbers against, although they may want to climb higher and smother the trampoline net too. Now there’s an idea 😆
Mountainous Northern Catalunya, Spain. Hot summers, cold winters.
You'll have to consider the sunlight too as most grasses are sun lovers and given the round shape of trampoline some parts will get a lot less sun if any!
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Try adding in some verbena bonariensis which will have similar height to those grasses and light airy stems with small purple flowers. They look great mixed with grasses.
You could also try some of the taller Michaelmas daisies would add some variety too. See this list which includes a few taller ones form 90cm to 1.5m.
https://www.gardenersworld.com/plants/12-of-the-best-michaelmas-daisies-to-grow/
Helianthemum Lemon Queen is another good tall plant with yellow daisy flowers in late summer.
Is it possible to extend the planting area out on one side? That would give you more scope to blend it in and make it look as if it’s a curvy design feature rather than an obvious disguise of a circular object that’s landed like a spaceship in your garden!
Another option would be some sort of decorative trellis curved around the legs that you may be able to grow some evergreen climbers against, although they may want to climb higher and smother the trampoline net too. Now there’s an idea 😆