First things first, I have pretty average memory, I don't forget important things too often, but I also can't instantly memorize things to the tiniest details like some gifted people can.
Around 5 years ago I decided to memorize my wallet seeds to have one extra backup method. My main backups are pieces of paper + a flash drive with wallet files.
I memorized all 4 wallet seeds (12 words each) that I have very quickly by rehearsing them from memory and checking with the written down seeds every day. For a long time, maybe 1-2 years I kept discipline and repeated the seeds every day, but eventually I started slipping and forgetting to do it, especially if I was busy with something. This affected my memory, and sometimes I forgot or wasn't sure about parts of a seed (2-4 words) so I had to look at it again and repeat it more often.
Interesting thing about memory, it's easier for me to repeat them in the sequence that I was always repeating (seed 1 -> seed 2 -> seed 3 -> seed 4) than to repeat only one seed that isn't the first. Also I can't repeat the words starting from non-first word or in an order that isn't original. So to me it's more like keeping in memory one big 48-word seed than having 4 seeds of 12 words each.
So in summary, I think memorizing a seed is easier than most people think, and there's no need to come up with songs or rhymes or other stuff, they can probably even make things worse. But in general memory is not reliable, even in long term, so this shouldn't be viewed as a reliable backup. It's just a nice extra layer of backup that doesn't require any complicated setup, and its strongest upside is that it's always with you (until your memory fails).
