Compression methods are given in the .zip file format, and set the decompression automatically if the program being used supports that method. Windows File Explorer seems to do only None and Deflate. This won’t bother you unless you open Duplicati backup files directly. If so, get a better program such as 7-zip which appears to support all the compression methods that Duplicati can do, and it can show you:
PPMd made the smallest block in my one little test of files just over 102400 (default block size), but it’s possible that the data (all files were Duplicati .dll files) just landed that way. If you want to benchmark extensively, feel free. I can’t recall (and can’t find) that anyone else has.reported results from changing.
Regardless, my test used all five options, a different one per file (files added over five backups), and the restore (with no-local-blocks set to force five dblock downloads to get source file block) worked just fine.