Congrats on doing this successfully, but your setup is amateur. This would have been infinitely easier if you were using IaC (Terraform/Ansible), containerized applications (that you're not already doing that is madness), and had a high-availability cluster setup in place already. It sounds like avoiding downtime is important to you, yet there's no redundancy in the existing stack at all, and everything is done by hand.

This isn't something others should use as an example.

Or just nix. The new hardware deployment would have been one nixos-build or colmena call, not accounting for the data migration of course.