Rescaling videos with FFMPEG for iDVD

I am preparing a DVD with several 10-minute videos in mp4 (produced from Final Cut Pro or Adobe Premiere), 1920x1080. I am using iDVD version 7.1 (1145) and macOS Mojave 10.14.6. When I export the project into an .iso image, it spends a few hours encoding the menus, and then a few more encoding the first video, and suggests 30 hours to encode them all. I stopped the process and used FFMPEG to encode the videos to a PAL DVD format, following https://www.kevssite.com/converting-a-video-file-to-dvd-with-ffmpeg/ :

ffmpeg -i input.mp4 -y -target pal-dvd -qscale 0 -aspect 16:9 output.mpg

I can add this video to the DVD, but it shows a warning sign and clicking on it shows a broken link: I tried other commands, such as scaling directly to 720p with -vf scale=720:-1 but it also shows the same broken link. I also tried [this script](https://forum.doom9.org/showthread.php?p=1809116#post1809116) , which does not produce an output because the setting for the bitrate is too low. How can I convert MP4 files to 720x576 for a PAL DVD with FFMPEG, so that iDVD encodes faster?