@string The procedure for generating "app" specific passwords is not necessarily made as clear as it could be, and on occasions in the past I have known it not to work quite as Fastmail intends.
I get a required app password " by logging into fastmail.com via a webmail browser, then going to Mail / Settings / Account / Password & Security / App Passwords, then take it from there. It's probably a good idea to have a Password Manager handy! At all times! Fastmail does provide an auto-generated password for you to use for the app but of course you will need to remember it...
I admit to being a fan of Office 365, including the 1TB OneDrive cloud storage and the other apps I think it's good value for money. And there's a copy of Access thrown in - OK so it's not the most glamorous database app on the block and there are security issues but it's very handy for personal use.
I totally agree about the thinking outside of the box, and in this case a change is much better than a rest for me at least. I've been forced to revisit all my passwords / usernames etc, and have got rid of a tonne of dead wood.