Models don’t have to be transferred. Extensions: download and install fresh copies. Do not copy them over from the previous installation unless you enjoy chasing load errors. Use the Extension Warehouse to automatically install extensions you got from there before. And the Sketchucation Extensin store tool to install your SCF extensions bundle.
Toolbars don’t transfer. You’ll have to position them. Wait until you’ve installed extensions so yyou can place all the toolbars in one session. I make a screen shot of the previous version and import that image into the new version so I have a reference for where to put the toolbars.
Custom materials and components folders can be copied depending on where you saved them for SU2018.
Written for an earlier version but the process I use is the same.