I did something similar for digitizing old camcorder family videos. I hooked up a VCR through a cheap elgato RCA -> USB capture card to my home server, and did something similar to you with a preview feed available on my LAN.

I even built a (really ugly) web UI which lets me pick an audio and video source, start the preview stream and display it in the browser without actually starting the capture, then start/stop the capture, edit the ffmpeg flags, and set the output file name. I can share it with you if you are interested