If you’re paranoid, you can convert the FLACs to WAV on an isolated computer (or VM), copy them and recompress them to FLAC or other formats. If there is a vulnerability in FLAC, it won’t persist through transcoding.
No idea about the site, never heard of it. Pretty sure FLACs couldn't harm your PC (correct me if I'm wrong).
Buuuuuut if you're gonna download another FLAC from the internet I think it would be safer to get it from either doubledouble.top, the Slav Art Divolt channel, or just Soulseek like the other dude mentioned.