Wait wait wait… how did you get the IPSW, if it’s not a National Security Top Secret?If you are talking about the IPSW of 15.7 for iPhone 8, then it is here
Personally I didnt experience any bug with neither 15.6.1 nor 15.7, but as always YMMV. I would say, why don't give 15.7 a shot when 15.6.1 is still being signed? You can fall back to 15.6.1 now if you got anything nasty with 15.7
I mean, I need to know it’s a link to Apple’s genuine servers.