I originally wrote this around the release of SU2017. It's still the method I use.
1. Custom shortcuts: Those get picked up by the new version of SU so that's automatic.
2. Custom materials, styles, components: Those folders from the beginning get put in a place I can quickly access with Open or Create a local collection and Add to favorites. (I never add my own custom materials, styles, or components to SketchUp's own supplied asset folders. They always go in folders I create.)
3. Custom templates: I just save out a blank SketchUp file from the previous version to somewhere I can access quickly. Open that in the new version and use File>Save as template.
4. Extensions/Plugins: I always install fresh copies from their source. This helps to eliminate potential problems due to changes in Ruby or other things that might cause old extensions to fail or throw errors. For me it isn't worth the trouble dealing with the errors if I know I can head them off in the first place. It also gives me an opportunity to do some house cleaning and skip plugins./extensions I don't use anyway. With the Sketchucation Plugin Store tool, it's easy to batch install bundles at least for files available there. The EW also makes it easy to automatically install files.
5. Toolbars: I make a screen shot of the current version showing the toolbars I have active and where they are along with the custom toolbars for the native tools. I import that image into the new SketchUp version as an image and use Zoom Extents to make it fill the drawing space. Then I don't need to look back and forth between SU versions to make sure I have things where I need them.