Cannot figure out how to delete storage space (Mac OS 10.11.6)

Please read whole post before commenting something snarky, I have genuinely been trying for hours to research and fix this problem and have gotten to the point where I just need to ask for help. I am using a very old macbook with the OS being 10.11.6 (To my knowledge this specific computer cannot be updated any further) Since I have trouble running a lot of projects on this computer, I ONLY use it for audacity and Final Cut Pro X (I make youtube videos on this computer basically). I have run into an issue where the computer does not run well because of storage space, and I genuinely have no idea what to do. I am currently working on a massive video project (and I cannot delete those audio/video/image files until the video is complete), but that is only taking up a small portion on the computer. I have deleted pretty much every other file and every other application off of my Mac, but it still says that I have 900GB taken up by "Other" in my storage. https://preview.redd.it/qf5j97ww8c7e1.png?width=615&format=png&auto=webp&s=fd7a066df4614a9e4a39135ed726750568dc6cb6 I saw people discussing that it could be caused by Icloud, and I have completely signed out of it in this computer, and that deleting the cache should help, but I have struggled to even find the folder. This os version is so old, when I look up advice all of the advice is for the newer macs and the settings look a little different and I'm having trouble figuring it out. I am wary of downloading any programs that identify/delete storage for you, as I don't want viruses and I only have a little space left anyways. If I could completely reset the computer from the beginning and have all the space completely freed up I would, but again I am currently working on a huge video project and I do not know how I would back up/save those files. Advice is appreciated. Please be kind I am a big computer noob but I have tried to research what could be causing that much space taken up in other and do not know what to do about it.