Well, DVD video uses a lossy codec (MPEG2), so the ship has already sailed for getting a lossless conversion of those tapes. Perhaps what you meant is that you don't want to do any further lossy conversions, which is understandable.
But that means that you will need to leave them in the MPEG2 codec. So you won't be converting the footage, you'll just be ripping it from the DVDs and putting it in a suitable container--you can use mp4 but I would suggest mkv.
Handbrake is a free, cross platform tool that will do this for you. If you do decide to further recompress to h264/h265 then Handbrake can also do that for you.
https://handbrake.fr
The nice thing about Plex (probably Kodi too although I haven't personally used it) is that it will transcode the files on the fly if you need to play them on a device that doesn't have an MPEG2 decoder. However, if you're running Plex on the NAS itself then you may need to check and see whether your NAS box is capable of hardware transcoding. If not then its underpowered CPU may struggle to transcode in real time.
Edit: Also check out
MakeMKV