It is not necessary to have an expensive machine to do good fMQ. It can be achieved on a home machine. To answer your question about thread built up. There is a certain amount that occurs when several points come together. I view this as whether it looks like carelessness rather than lack of control. I don't see your build up as effecting the sale of the quilt but other things. They are.....difference in the quality of your stitches. The quality of the stitches in the vine look good. They appear consistent and smaller. Those in the top picture are too large. They too are consistent in size which indicates good control but too large. I would rip these out. The other problem is the inconsistency of the shape of the leaves. This is distracting. You have several good qualities to your machine quilting. With practice in better control of the work you will definitely improve.