show an overall picture.

And check out the conifer and shrub forums.

Another shrub combo in the yellow and burgundy tones are Coleonema and Cotinus - though the Cotinus has a short deciduous period.

Berberis in certain areas is not considered invasive, if its not invasive in your area it might be used as a burgundy shrub and combine it with a yellow carex morrowii.

I'll try to attach a link but not sure if it will work. showing a burgundy cotinus and a yellow japanese forest grass against a dark green shrub background .

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If the heavy soil is killing the flowers buy lighter soil and dump it on top, without mixing it with the existing soil, plant in that. Or choose adapted flowers. You will have to do the same with shrubs, if the kinds you want don't like clay soils they won't be any happier with the existing soil than the flowers were.

Is there a hedge, wall or solid fence behind the border, as in the above photo? If not you should install such or plant evergreen shrubs behind, informally arranged, as well as at corners or ends to in either case form a sort of stage for the smaller plants (groundcovers and dwarf shrubs, if no herbaceous plants are to be used). In the well-designed mixed border deciduous shrubs serve primarily to form transitions between the bulkiness of evergreen kinds and the lightness of non-woody plants. Flowering climbers like clematis and roses can be trained on fences, walls or hedges, to include them in the planting, as well as large deciduous shrubs like lilacs that provide no summer interest otherwise.