Missing files when moving projects to portable drive

So I have a 14" Macbook Pro with the M1 Pro chip, 16gb Ram & 512gb ssd.

All I do is make videos on imovie and obviously storage becomes a problem. I want to keep my old projects so if I need to, I can refer back to them at anytime and grab clips from them. I usually have a music track in the background of all my videos, so being able to go back and grab an old clip without the music track to use them in newer videos is the main reason I need to do this. So in the past, once my mac started to run desperately low on storage, I would export the library to a portable ssd drive. This has always worked for me in the past, but since switching to filming in 4K, my storage started to run out extremely quickly. I started to work directly off of the portable ssd drives, so I could build up larger libraries before starting the next one and that was working fine for a while. Once the portable ssd started to get quite full, working off of the portable drive became a nuisance because imovie would start lagging while editing. And now, even editing a new project on an empty ssd drive seems to be a struggle on my mac. So I came up with an idea to start editing the videos on my mac drive and once I was finished with it, I would then use the "move project" option to move that project to the imovie library that is stored on the ssd drive. This sometimes works and sometimes doesn't. I have been finding once I have moved the project to the portable ssd, I will then open the library that is on the ssd and see that some files will be missing and a large number of the project has not been copied and now the project has a missing file and technically, is not backed up. The only thing I can sometimes do to save it is, if I click and drag the original video file from my mac, into the project, then it will copy to the library of that project. If the imported clip that is missing came from an sd card, then there is no way to get the clip back into the project.

When trying to find a fix, I read some people say you have to choose the "consolidate project media" option, but every time I've tried this, every project will say that there is no media to consolidate and I'm assuming it's because the files are already imported into the project. My imported clips come from two sources, a sd card from my camcorder and also videos from my iphone. I import the sd card files straight from the sd into imovie, while the iphone videos get airdropped onto the mac and then copied into imovie. There is nothing specific about the video files that go missing in the transfer process. Sometimes phone videos are missing, sometimes the sd card videos are missing, sometimes I have no issues and everything copies over fine.

Does anyone know why this might be happening? It's becoming very frustrating that I don't have a reliable way to back up my imovie projects. I appreciate anyone who's taken the time to read this and may have any advice. Thanx.