NixOS 26.05 on an older AMD laptop: boot errors, failed downloads and eventual success

Hello everyone, I’m documenting my first NixOS installation as part of a practical project to develop my NixOS skills and understand how it performs on older hardware. I’ve now managed to install NixOS successfully, but the process involved several problems that I haven’t fully explained. I’d appreciate any advice from people with experience of NixOS, particularly regarding the initial boot failures and installation errors. I’m sharing this as a troubleshooting report rather than a solution, ...