If you are currently using 490GB, 512GB will still not be enough. Running a computer with less than 50GB of free space will not leave room for many maintenance, OS update, and other caching needs.
Generally, the items that consume the most space are media items. Things like movie libraries and music libraries can easily be moved to external drives. There are also any number of things that you likely do not need on a daily basis and can be moved to external drives. These generally do not need super fast external drive access, so you will generally see no difference with these files on an external drive vs. an internal drive as in the case of media files their streaming rate is much lower than data transfer rates on external drives.
Having tons of free storage no matter where it is located still does no help in terms of system RAM which is required for the computer to actually function. How much RAM you need depends on how you use your computer. If you have lots of apps open, using apps that require lots of RAM, or have tons of tabs open in a web browser, the more RAM you have the better.