It is possible to quilt a large quilt on a small harp; it just takes practice. I do it all the time.
If you enjoy the learning process, and it sounds as if you do, get yourself a sturdy used machine that is all metal; in other words, a 'pre-owned' older model. There will be fewer things to go wrong with it. Then start on small quilts and gradually work your way up the sizes. You will gain both confidence and skill as your finished pieces stack up. And I can pretty much guarantee you that as soon as you finish a baby quilt, you will find someone who needs one!