Just looked at the
manual for the DXL603 and you're absolutely correct which I find a bit odd but such is life!
Case in Point, the Janome 400 hasnt got an openable Face cover or adjustable presser foot pressure which I find very odd for a £500 machine at the time of release ... which is why I quickly upgraded from it!
In the manual (page 39) it says Stitch 20 is a decorative stitch so obviously their version of a Serpentine stitch,
I agree with Acorn that trying to lengthen the stitch length might give a more gentle wave ... try it, noting down the numbers as you go.
SID is not only boring but damned difficult too ... Echoing the seams a walking foots width away is a lovely solution as is crossing diagonals (on Square and Rectangular blocks and more)
Look what a lovely pattern has been made by crossing the diagonals
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and this is the front
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I dont know how old your friends machine is or if its brand new but Janomes certainly hold their value and can be sold on for 50%+ of the retail price if you decide to upgrade in the future.