Next time write it down on a post it note and stick it in a drawer. Learned that the hard way myself after losing access to a ten year old gmail account.
I have a notebook that I write down all appropriate info regarding passwords, supplemental security information, etc. I often let the OS set up a secure password, but there are some organisations, like the local government, that requires a password to conform to their specific requirements, which is a pita. Apple’s new password manager is a lot better to use than the old one, I’ve got to have a run through a delete a bunch of possibly compromised ones, but they’re all sites that I haven’t used in years, and don’t share with anything that’s current.
FWIW, my passcode on my phone is 6 digits – I’ve considered an 8 digit or an alphanumeric one, but an 8 digit would probably be simplest but very difficult to hack, if not impossible.