Possibly just use the one from Brotli since it's already standardized. If it's any good then the work is mostly already done, right?
Personally, as someone who doesn't work in web, I'm just as happy that zstd is flexible this way. For my applications, the brotli dictionary is pure overhead that bloats the library.
They want _every zstd decompressor_ to __already have__ the dictionary in question so that it can be specified as part of the standard. E.G. 'instead of empty / an initial in file dictionary, use the standard dict #3' Such reference dictionary starts would be not be included in .zstd files, but would be shipped with the compressor source code.