I leave everything in the library on the raid except backups which by default are in the system drive ( fusion drive). My raid should get better performance than the system drive, plus wouldn’t the cache just be eating up space on the system? Another reAson not to have the library or anything of any size on the system. My time machine also backs up my system drive and I don’t need it backing up cache files.
I do leave files in place quite often, but by the time a project is done and all cleaned up for storing away, I consolidate all the files into the library.
It sounds like the op was still keeping the library in the user movies folder. I would move it out of there. Even if you are keeping it on the system drive I’d keep the library in a folder with other elements for the job like motion projects, psd files, AE projects, etc.