i recently converted my music library (approx 12,500 songs over several hundred albums) from ALAC to FLAC using Foobar 2000.
It took several days, because Foobar doesn't support batch conversion, so will only convert the tracks imported into it from each album.
Basically I opened two windows in file explorer (one for the ALAC source and one for the FLAC destination and Foobar.
I copied the ALAC files (including Artist and album folders into the FLAC destination, thern deleted the ALAC files in each album folder to just leave the artist and album folders.
Then artist by artist, album by album, I copied the ALAC files from each album into the Foobar window, selected "all", "convert" "FLAC" and then selected the destination folder.
Other than being very time consuming, it worked very well.