I don't know if this is true for others, but the way my playlists come into existence basically involves me playing a lot of different music from a whole bunch of places and if I think one random thing sounds good with another thing I have them play next to each other, and this process repeats slowly over time until the playlist gets pretty long (50+ items avg) and everything is in the order that I like it. Very often stuff is saved off YT or SoundCloud and put in various folders likely to never be sorted.I don't understand your comment... the steps I described do exactly what you want at first post, with the original order you set on playlist (?). Obviously you have to enable EXTM3U as noted.
It's fine when everything is accessed from one PC but eventually there comes a time when I want to share my playlists or play them on an entirely different PC. Ideally I would just copy the files and m3u to a flash drive and when the m3u is accessed on the flash drive everything just plays in the expected order. Even if foobar can treat absolute paths as relative if the actual files are indeed in the same directory as the m3u there's no guarantee other programs will do the same, which could be a problem down the line.
Perhaps a simpler thing I could also do is just modify the m3u playlist directly to replace the absolute paths with relative paths, but that would still involve a separate text editor and likely a bit of regex, so it's debatable if this would be faster than just sticking with M3U-Copy. Or would your approach be faster at making my playlists portable?
The Playlist Manager too, along many more features and utilities which may come handy for syncing playlists between different PCs, a shared music network server, etc.
If any of those solutions are good enough for you, that's something you will have to try by yourself.