It's a shame that you have left it so long, but even so it is still less than a year old and should be under warranty.
A RIB your size should not need hydraulic steering. Some might prefer a hydraulic system, but a properly installed no-feedback cable system should be completely adequate.
From your description it sounds like it is not properly set up. I would take the boat back to the dealer and ask them in a fairly robust way to sort the problem out.
Let us know what happens and if they are unhelpful we'll suggest some ideas for plan B. Needlessly spending several hundred pounds extra should be a last resort -- I'm assuming that you haven't got unlimited funds as you'd probably have a bigger boat!