There are third-party apps available that handle FLAC just fine. I use a paid app called Neutron Music Player mainly because it has an SMB client to connect to

There are third-party apps available that handle FLAC just fine. I use a paid app called Neutron Music Player mainly because it has an SMB client to connect to a NAS with my FLAC collection, but you can also use its built-in server to transfer FLAC to internal storage. I think you can even sync the SMB source to local storage as well but I haven't explored that, since I play them over the network just fine.