I think you may need a successful switch to actually apply the addition of the proxy CA to your root CA store before attempting any other changes that could require reaching out across the network. At least that was the order of operations I had to follow to remove an offline remote cache before attempting any package updates.

You should be able to rebuild offline if the minimal change set is self contained enough, as in purely local. Did you update any other inputs? I guess you could be missing some kind of extra TLS or CA store dependency for adding custom CAs, but that doesn't seem likely for regular NixOS install. I use flakes instead of channels, so I wouldn't know what else may be blocked you. A stdout log may showcase your error more clearly.