Surely, this was a material issue.
If the engine version resolved it, then your snow may be using something the engine has control over like RTV or similar which is a huge red flag to any serious developer concerned about optimization.
The scene looks good, if you know you no longer need to work or alter it you should bake it all down.
Produce meshes, and only use meshes.
Even if not wanting to use meshes, you should still be using 4k maximum size tiles or even smaller since everything that’s not a 4090 is going to have issues with the timings otherwise.
Ideally, all landscape textures need to be 1920x1920 at most, including the layer paint which unreal can only do by scaling the labdscape up and introducing a boatload of other issues (so don’t go scaling landscape).
The way around it is again to bake it all down to meshes where you can easily scale the single texture material up or down as needed.
Anyway, good luck with ue5 in general 
For those who cannot read between lines: you shouldn’t be even trying packaged build with the basic landscape.