Burning to a disc shouldn't be a process where you files are altered in any way, ideally. You should create an actual DVD project (ISO or folder) so that you have a master. That way, when you burn discs you are essentially just copying the video over, whatever it looks like on your PC will be how it looks as a DVD. You can also make some cool menus. However, if that's not the problem and your video looks bad even before burning to a disc, then that means you need to export with a higher bitrate and use larger DVDs that can hold more space (quality).
Also, I'd recommend editing the Blu-ray version of the film so that when you export to a DVD resolution there will be far less quality loss. You start with low quality, you're always gonna end up with low quality unless you crank up the bitrate really high (but then you won't be able to fit it into a disc). I'm sure there's a middle ground though, 1) make sure when burning you are not transcoding 2) use the highest bitrate that will fit on a DVD, less than ~8GB for dual layer