Take any old drives you have laying around and copy & paste the Photos library to them. Put a sticky on each so you can easily remember the date you last backed it up to each drive or use a unique identifier of each drive and keep up with a note in Notes on Mac. Even ancient, retired drives that still work can still do a basic job of one more backup copy. Regularly update these backups on some sensible schedule that fits how fast you add a fair number of new photos to your Photos library.
If you lack old drives, buy upwards of several equal to the current size of your Photos library times maybe 2 or 3+ (for future Photos library growth). Then do the above with those several new drives. I suggest bare drives and then use a HDD dock (
one of many examples) for this because it is generally cheaper than buying externals already in a branded case. There are
cheap plastic storage cases for bare drives too.
Copy & paste the photos library onto your two+ TM discs (you can put files on the same drives; they just eat into the total space TM can use for TM backups) and then update the library copy with each rotation. In other words, before you do the next swap, copy & paste the current library to the drive about to head offsite. And then just make that a step to do just before each drive rotation. You'll essentially be a doing a manual backup copy of one file (though it's actually a whole library).
If you have more than one Mac, including an old, retired one, that still works, copy the library to the other Mac (if it has capacity). It doesn't matter if the old Mac is unable to open the library, you are just using it as spare storage.
If you have a NAS, regularly copy the library to it too.
If you have or can borrow a BD optical burner, export photos from Photos to folders up to nearly the maximum capacity that a blank disc can hold and then burn you a stack of BDs. That's at least 25GB but can be 50GB, 100GB or 128GB per disc. Photos are generally "forever storage" so a permanent burn of files you want to keep forever is a fine use of optical (write) technology.
Get a stack of discs on a spindle and then store the burned discs on the spindle for an overall small package... ideally stored
offsite. Each time you have another new block of 25GB-128GB of new photos, burn one more disc and add it to that spindle.
Just like with TM, more backups are better than fewer... and getting at least ONE backup
offsite significantly improves recoverability in fire-flood-theft scenarios.
Cloud storage is always one more option. I don't like the idea of putting my media on "strangers" HDDs "in the sky" myself, but it is one more place to stash photos that is already "offsite." There are some cloud services that sell a block of storage for a "lifetime" price. So you could buy one of those, copy all of your photos to that storage and have one more "offsite" backup, albeit at the mercy of anything that can happen when you trust total strangers with your private files.
Another "precious" kind of media is home movies. Same suggestions as above. And if you have to buy discs vs. using some already on hand, consider home movie capacity times about 2 or 3+ too.
