I'd like to back up these items before reinstalling VS. Where can I find them? I found an answer saying that this data is stored in the registry, but it doesn't seem to be the case for VS17.
VS2017 no longer uses the actual Windows registry, and uses a registry file stored at %LOCALAPPDATA%\Microsoft\VisualStudio\15.0_<random-characters>\privateregistry.bin.
You can load it as a hive in the registry editor if you want to look into it to check for the start page contents. Otherwise simply try copying and restoring that file in your new installation.