If it works then that’s alright. If you do have issues in the future, just keep this option in mind.
Oh, well everything is mostly markdown. You can also see this guide on formatting in discourse if you’d like to know more.
You only have to manually specify the version if it’s not available for your system or if you’d like to keep the drivers pinned to a specific version. From this list, we can see that the 555.58.02 drivers aren’t available for NixOS 24.05.
Generally, stable releases are always outdated compared to the unstable branch of NixOS, that’s why we have to do this ourselves, but once this change is rolled back into NixOS 24.05, it should automatically be applied after a system update.
Now that I’m writing this, though, I’m noticing an interesting thing. For NixOS 24.05, we can see here that the latest stable version is 550.54.14, which should be the default according to the hardware.nvidia.package option definition. However, your system is trying to build 550.78, which corresponds to the production version.
Could this be the cause of the initial error, perhaps? 
Edit: Probably unrelated. I think this comment explains the problem pretty well.