How do I force Safari to save password for ALL websites?
In some cases it is the website itself telling the browser to not save the passwords. There are a number of ways a web programmer can implement this and a number of ways to get around it.

I used to use a little JavaScript "bookmarklet" that tried to turn off any JavaScript on a website that told the browser to not save the password. It worked about a quarter of the time. Barely worth the trouble.

More recently I have moved to a 3rd party password manager (1Password) that will save the password for any website.

Editing a preference file is chancy and annoyingly "manual" but possible (EG worth looking into if it is possible) depending on how geeky you want to get. And checking for preferences that say not to remember a password for a specific site in Keychain or Safari itself is worth checking on. But my bet is on a website telling your browser not to remember passwords.