Yes, it's best to have a TM drive that is at least equal to the total of your internal
and your external/s combined. The bigger, the better, as you'll be able to keep backups for that much longer, then TM starts auto deleting the oldest. For some of my backups, I'm using a pair of 5TB Seagate's from Amazon that cost just €120 each. I now have 3 TM backups of all our stuff kept in 3 separate locations.
Just be sure to include the extra drive/s in your TM backups. With the external drive containing your photos, music etc attached together with the drive for Time Machine also attached, open the main computer settings > time machine > options. Then, by default, you should see the external in the exclude list. Just remove it from the list and it will now be included in your next TM backup. I think excluding externals by default is a very good idea, because you don't want TM backing up a drive you might borrow from someone else. You will see the drive 'Sabine 4TB', that's my wife's drive, which is backed up by TM on
her computer. When TM is finished, you can browse the folders on the backup drive to verify that the external has also been backed up.
View attachment 1815716
View attachment 1815717
You didn't exactly ask, but if I may, I would caution against using cheap HDD's for certain things, like huge photo libraries, unless you have time. Pay attention to the read/write speed of the drive. I learned my lesson the hard way. Our 700GB Photos library was unusable on a regular drive, but at 1,000MBps, the speed of it on the Samsung T7 makes my eyes water. A winning combination together with the M1. Oh, and the physical size of the Samsung SSD is tiny. It's as big as a credit card, as thick as a pencil and weighs only 50grams. A perfectly portable setup. The old HDD is now being repurposed as an extra TM backup and will be left at home.