I'd look at some of the creamy/rusty coloured echinaceas and kniphofia and some plummy-brown colours ... Hemerocallis 'Summer Wine' is a good one. The blue-leaved hosta 'Hadspen's Blue' looks good with those colours and gives a bit of a zing.
“I am not lost, for I know where I am. But however, where I am may be lost.” Winnie the Pooh
I think Bergamot would swamp the chocolate Cosmos. They are quite delicate plants that don’t get that tall.
Something like Verbena Officinalis Var. Grandiflora ‘Bampton’ that has dark stems would pick up the dark flowers and create a more delicate airy feel for added height could work well.
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The blue-leaved hosta 'Hadspen's Blue' looks good with those colours and gives a bit of a zing.