Effective but simple'ish sound set-up advice

Hi, I have just joined the forum so hello to all and please be patient if I ask dopey questions.Thanks.
I have a Sony HXR NX5E camera and have been up until now recording sound directly into the camera from my Rode NTG3 Mic either camera mounted or on a pole. My question is this- should I be looking to use something like a Sound Devices Mix Pre in between or continue with my current set up? I am embarking on a Documentary which will be predominantly a one man band operation. I'm a Photographer by trade so tend to concentrate on composition of image as oppose to sound but realise the importance of sound quality over image quality and want to get it right without complicating things to the point that the product becomes worse due to spreading myself too thin over several jobs.If anyone could advise the best route that would be great. If I use the Mix Pre is all my sound quality now down to the mixer instead of the camera sound card which now just acts as a recorder. If this is the case why would you use a Sound Devices 702 for example with a Mix Pre instead of just a cheap zoom recorder with the Mix Pre?
Sorry for the protracted questions but i'm just trying to get the best quality gear right before I get half way through filming and realise my mistake. I always try to get a real handle on what i'm doing before I do it so I can concentrate on the creative side as much as possible.

Thanks,

Aidan