@Christoph-Hart Of course. Inno Setup also lets you do it with a bit of scripting seeing you already have the variable.
But if the user moves the library after install, you need some sort of mechanism for adjusting to that and the baseline is Kontakt which lets you pick a folder, it searches the entire folder and all subfolders and you can set it to use the folder you selected for all other attempts in a session, automatically correcting all missing samples no matter the developer, if you e.g. moved your entire library collection to another place.
If they load up a session with 20 instances of my instrument, I don't want them to have to close the entire project to reload the samples (your built in solution requires an instrument reload; all you have to do is call Engine.reloadAllSamples() if the folder contains the samples).