Did you use the Time Machine drive from that Late-2013 iMac with Migration Assistant when setting up the M1 iMac. I looked at my Keychain on my M2 Mac mini Pro and it has some really old content from years ago that suggest the keychain data travels with Migration Assistant.
If you were using Time Machine, and did not reuse that Time Machine drive from the 2013 iMac, then your document may exist on it prior to your setting a password, or even in another document format such as PDF.
I am uncertain about what Keychain data is shared with iCloud (if any) and if you cannot open your Pages document from iCloud drive, then that theory is out the window.
Your first bullet:
Write documents to a unique folder on a DVD. Both in Pages and PDF format. Include a plain text file in that folder with the password. Print those documents on a laser printer and enclose in a waterproof sleave.
Store both either in a home-based fireproof safe, in a bank safe deposit box, or with a younger attorney whom you appoint as executor of your estate. You also might consider drafting one of the available trust formats for when your luck runs out… I used to travel extensively on business and had my attorney draft a non-revocable trust that would take care of my parents and the "pour over" will had my attorney as initial executor and the bank to manage the installment payment terms of the trust.
Second bullet:
Digital and printed content stored in a bank safety deposit box.
Unless you are a member of the bar, don't be your own attorney in the above matters. Seek professional advice.