Your method could be described as secure as those who generated their seedphrases online but wrote them on papers without memorization.
People tend to forger they are like computers with natural/organic internet. Memorized phrases could be hacked if you are connected to corrupted humans who are connected to a dark entity that troubles the human society. It's like a robot that stores information in its memory without realizing it's easily accessible to the controller of the super-computer or datacenter it may be connected to.

I think it's better not to memorize your seedphrase unless you only form strong connections with people that are trustworthy. And there is the possibility of someone gaining access to your mind without the connections. They could just do it wirelessly via  network masts, from space, or physically by attaching something on your head.