Looking at your zulip post, it seems your problem is not really with the precompilation of GLMakie but with the package manager. The fact that you get an error when calling instantiate has nothing to do with the sysimage compilation. It is more likely due to some issue with your julia configuration or package cache.
I’d suggest maybe removing (or just renaming) your .julia folder (where julia stores all downloaded packages and default environments) and try rebuilding the environment from a basic Project.toml containing only GLMakie at first. If that works you can add PackageCompiler, Test and the other modules I put in my precompilation script.
Alternatively, you may want to consider these slightly simpler precompilation instructions https://github.com/JuliaPlots/Makie.jl/issues/799#issuecomment-752599938 which require fewer packages in the environment (but might also precompile less stuff).
Finally, do follow @sdanisch’s advice and make sure you do not add MakieGallery to the environment. It is not compatible with the recent AbstractPlotting changes and will hold it back to an old version. The version I have in my dev dir was modified to test if it might be compatible with little modification (not the case). Sorry for the confusion. I have modified the environment files on github.