Short update:
While writing this, I found the thread “Plots.jl, GR.jl broken due to GR_jll v0.69.1+1” (unfortunately I am not allowed to link, but you find the thread also here in General Usage on the first page)
There it says that GR_jll v0.69 was broken and v0.70 is fixing it.
I was removing the restriction of GR = 0.72 in the Project.toml and using v0.73, however this generated an error during PackageCompiler with the package Qt6Wayland_jll, no idea what is wrong with it and I wasn’t in the mood to look into it.
So I thought why not test the fixed version v0.70, so I restricted GR to 0.70, PackageCompiler worked without errors and also the svgplugin.dll was found! (This time, the svgplugin.dll was not searched in the whole path, but only in the most important system directories, and then it was looked into every single artifacts\123hash folder until it was successfull in the correct artifacts folder…
So to partially answer my question: In v0.70 julia did not know in which folder to look for the dll and just tries every artifacts folder. This was apparently changed in later versions, however it does not seem to work correctly or consistently.
For now this “workaround” with using the old v0.70 seems to work. At least until some incompatibilities with old versions of GR arrise…
Edit: Links seem to work now. See here for the refferenced thread: Plots.jl, GR.jl broken due to GR_jll v0.69.1+1