I think you invented (or reinvented) a simplified password manager, or a plain-text password manager. A usual PM solves the task of managing by human memory unm

I think you invented (or reinvented) a simplified password manager, or a plain-text password manager. A usual PM solves the task of managing by human memory unmanageable: plenty of passwords with variable complexities routing them all to a single one intended for the human. In your system you have your own version of the master password (prefix + suffix) that locks out your actual password (a single substring). There are obvious drawbacks compared to a general PM like a much lower space of possible variants or needing to manage this manually (like generating the file or choosing randomly your prefix and suffix). But there is at least one benefit, if you keep the system simple enough (while not making it simple in generating the sequences), no hidden vulnerability should waiting to happen