border quilting:
How about zigzag or a Greek motif...squares
To sort of play off what you did with the white strips in the corner?

something like this below but the next section would have the horizontal line at the top.
This is a pretty familiar Greek motif but have no clue what it is called...Should be easy to quilt because it would be continuous.
and since your strips are narrow it could be adjusted easily.

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I would continue the triangles you have in the main sections.