I store my seed in txt flle as well, and this file is in my phone, and noone can read it (except me). It's very easy... I have packed the file in 7zip with a password, the password is more than 20 characters long.
Have you personally reviewed the code of 7zip to ensure there are no flaws in its encryption algorithms?
Did you take steps to mitigate against known vulnerabilities such as this one:
https://twitter.com/3lbios/status/1087848040583626753?
Did you make sure to build the app yourself from the source code you reviewed to ensure you haven't downloaded a fake or malicious one?
Did you make sure to delete all the temporary files it creates in the archiving process, and then write over those sections of your phone's memory with junk data?
Did you make sure to delete the unencrypted text file you would have first loaded on to your phone before encrypting it, and then write over that section of your phone's memory with junk data?
Do you never unencrypt it unless your phone is offline, and then again delete and over write the temporary files it creates?
There is a reason that everyone tells you to write down your seed phrase on paper and keep it offline.