I would upgrade to Mojave for now. You can always upgrade later to Catalina. I myself run Mojave.
I use WD My Passport eternal Hard Drives to store my iMovie libraries. I have read that the OWC Mercury Elite Pro 7200 RPM drive is good, too, but have never tried it myself. As for SSD drives, it is unclear to me as to whether they are suitable for storage of an iMovie library. Apple recommends against storing an iMovie library on an SD card or a USB Thumb flash drive. So not sure whether an SSD external drive that is not a Thumb drive would work. I wasn't able to fine anything on Google about it. Since to my knowledge Apple has not explicitly approved external SSD drives for iMovie library storage, for safety I don't want to recommend it to you. However, I believe that OWC has an SSD external drive that you can check out. They are pretty pricey compared to Hard Drives.
External drives can, and eventually will, fail. For that reason I have two WD My Passport HD 2 TB drives and store double copies of my iMovie libraries on them. I have an additional 1TB drive dedicated to my Time Machine backups. So I am triple backed up.
-- Rich
-- Rich