Agree with most replies. Have owned a JD 200 series and two different 300 series and all mowed well if set up correctly. Appears your dealer did not set up the mower correctly. (Deck, steering, etc.) Any unit just out of the box will need to be properly set up at the dealer to operate correctly. 4WS, if you think you need it move up to a zero turn. Gardening? You will be better satisfied in the long run with a good quality tiller, not an add on to a "Mower".
Lots to consider. As stated tell your dealer your concerns and if he does not correct by setting up the mower correctly....return it. Whatever you do don't step down to the 100 series. You think you are unhappy now, with a 100 series you will be VERY unhappy.
Regard,
Gil
JD GX345
Gravely ZT HD-60
JD 2030
TroyBilt Horse Tiller
JD Gator
Many accessories and implements