Since feed dogs UP makes the stitching better, could there be a problem with the darning foot not pressing hard enough?
If you have a local dealer, try out the janome 1600p. Side loadig bobbin, straight stitch only. It runs about $600-1000 in the usa. Crazy as it seems, it may make sense to have one machine for sewing/ quilt assembly - the high end machine - and another straight stitch only machine for FMQ.
You could even park yourself at their machine and keep going until your project is done - test it thoroughly!