Working in batches if necessary, cook the bacon in a large skillet over medium heat until crispy, about 10 minutes. Drain on paper towels.
While the bacon is cooking, grate the cucumber on the large holes of a box grater. Wrap the grated cucumber in cheesecloth and squeeze to remove as much liquid as possible.
Combine the cream cheese and mayonnaise in a stand mixer fitted with a paddle attachment and mix until smooth, about 1 minute. Add the cucumber and red onion and mix until combined, about 30 seconds. Season with salt and pepper. Cover and refrigerate for 1 hour.
To assemble the sandwiches, divide the cream cheese mixture evenly between 4 slices of bread. Break the strips of bacon in half and top each sandwich with 4 halves. Sprinkle the borage petals over the top, then cover with the remaining four slices of bread and press down firmly. Cut away the crusts. Slice the sandwiches diagonally in half and then diagonally again, to make triangles. Serve.