There are no “artifacts” in a Natress PAL conversion.
There can be if you use compressor.
The slightly softer picture has to do with the picture being enlarged nearly 100 pixels.
720×486 becomes 720×576 and a slightly different pixel ratio. In that regard it makes perfect sense that the picture would be slightly soft.
the only thing that could make it better is an expensive hardware device like a Teranex.
fyi