300gb free on a 1Tb drive is quite adequate, at least for now. If this free space is diminishing then you will need to eventually take action. As Fishrrman correctly points out, the only fix is either adding additional storage space, either an external drive or replacing the internal with a larger drive, or curing your "addiction" (reducing your acquisition of new files and/or culling older files). For most of use, adding storage space is the only realistic answer.
The OP needs to research methods of moving the files to an external. How fast and aggressive that research needs to be depends on how fast there drive is filling. I it indeed is filling "slowly" then they have time to work on the solution, but work on finding one should not be ignored.
I don't know iPhotos/Photos or iMovie so I can't say whether they can use libraries on an external drive or not. If not, then the solution would be to either "retire" some older files from the managed libraries and archive them on an external or to move to using different library management apps.