I think you need to devise an overall backup schema, which would depend largely on your level of crazyness/paranoia/assessment of the odds of something bad happening. I've not otherwise seen the assertion that an SSD should be always plugged in, but that's not to say that's not true. So assuming it is true, any offsite backup should reside on EHDs. You also have to figure out a backup schedule that would fit your lifestyle.
I have my music on a 4 TB SSD, with a backup drive on another 4 TB SSD, both of which remain plugged into my iMac. My backup software (SuperDuper) is set up to do an automatic nightly backup from the main drive to the backup. I also have a 4 TB EHD which I plug in monthly, perform a backup, then disconnect and store somewhere. Finally, I have yet another 4 TB EHD to which I backup twice a year and store in yet a different location. I've debated whether that's overkill, but decided that, considering how much I paid for that music and the work I put into ripping the music (or downloading) and correcting the data, it's prudent.
Here's how I view it: Let's say a music file corrupts on my main music drive (don't know how it happened; let's just go with the possibility). Overnight, that corrupt file will backup onto my backup drive and corrupt the copy there. However, if I catch the problem within the month, I can access the monthly backup. If I don't catch it within the month but I do within 6 months, I can access the bi-yearly backup. I've therefore eliminated most (not all, but most) of the risk of losing the data.