The purpose of external runtime folders is to reduce the startup time of your application as well as providing some structure and organization of your content.  Like the others, I have several external runtime folders under various categories for easy access.  The base characters and their morph packages reside within the application's folder; content like clothing, characters, poses, props, etc each get their own external folder as needed.  Use a program like 3D Content Installer to keep track of your content installations from zip and exe files.  It is an excellent application which can easily uninstall the content you have loaded.  Word of caution though,  the zip content must have the same hierarchy structure as the runtime folder for proper installation.  It is wise to inspect content carefully.

Victoria_Lee made an excellent suggestion to initially create some type of database for your content.  You will find your content growing exponentially as you collect.  There are some free databasing programs available which you can use; 3D Content is a for purchase program from DAZ that also keeps track of web links and stores images of content.  Hopefully the bugs have been worked out with the latest version.

If you want to reduce your runtime overhead a bit, check your content for unnecessary files and folders.  Some older content have .rsr files for image icons displayed in the library.  The convention now is to use .png.  However, some content include both. You can use the free version  P3DO Explorer to delete or convert .rsr files.  Word of caution, there is an rsr file that resides in the Poser folder; DO NOT delete this.  It contains essential data for the program.

Finally, there are individuals carrying a tremedous number of runtimes on their had drive which, IMHO, is completely overkill.  You do not have to install every piece of content you own.  Many will be used once or twice then forgotten soon afterward.  Be selective, install what is necessary at present and archive the rest for later use.  I have a Projects folder just for that, projects.  I install content, create my scene, render and then remove as soon as I'm completely done.  It reduces my content load and I don't have to keep track of it again until the future.

Hopefully this services as a guideline for your return to Poser.