(One thing worth noting: the newer APFS-format OSes cannot be backed-up on HDDs, due to how they're designed to run on SSDs. I'm not impressed with Catalina, and dissuaded from using it by all the breakage of plug-ins and such. I moved to Mojave from High Sierra mainly to go easier on my eyes with the dark mode, and everything I use survived the move except for Best Service Engine (K'd), with which I was using the Synth-Werk library, fortunately, I'd kept a partition of High Sierra on the SSD, with Synth-Werk still available to me in Logic there. I also have one of the USB ports occupied by a 256GB shorty USB drive, where my libraries go (Arturia, Gforce, Spitfire LABS, Waves, Battery, Reaktor) rather than on my system drive.
HDDs are good for recovering lost data with Disk Drill, I have been able to recover a good 99% of data from an "erased" HDD. I myself replaced the optical drive with a 7200 RPM HDD for data. Those specs are the same as my Macbook Pro.