The flowering crab blooms the same time as my apple tree, in May.
I'm expecting delivery of a salmon pink hybrid peony,which will be put just at the edge of the flowering crab's shadow.
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Note the similarity in coloration with the that of the crabapple's.
May I suggest a flowering quince - or two - as a companion planting for the mugo pine, should you decide on one.
chaenomeles
It should provide a delightful and modest-blooming ground cover. It's usually trained to be a low,ground-hugging shrub, not fully grown as shown on the Google page.
I hope my suggestion is helpful.
*edited - shrank the picture a bit - nn