Something to try - repeat the test with a typical quilting cotton instead of linen. If that looks ok, try a lightweight fusible on the back of the linen and repeat the test.
Linen has a looser weave and a tendency to stretch. It could be that the fabric stretching during stitching and relaxing back into place afterwards is what's making the stitches go more "wonky" than expected.
That said, machines capable of decorative stitches will always have a slight angle to them. If straight stitches are important, a straight stitch machine will have the advantage.
Juki's TL series might be worth considering.