If you do not wish to write your passphrase in anywhere in fear of compromising it try to make a rhyme or poem with it only for you. Mnemonic phrase usually some random word so give them a meaning so that you won't forget and do not tell it to anyone. Even if you write it down in some place no one will understand what is it if you do not tell them.
It is not an option between writing it down and memorizing it, seed phrases were meant to be written down, that is also one of the reasons that 12 words phrases are used, cause they are easier to write down while providing sufficient security.
Think of how many song lyrics you could comfortably sing word-for-word some years back, try them now and check the accuracy level. Even if you could sing the song perfectly, that is only for how long your brain is working efficiently, if any accident or trauma affects the brain you may lose access to that memory. Aging of the brain also deteriorates its performance.
- Jay -