Per RFCs, "example.com" (the bare domain name) must be a DNS A/AAAA record and cannot be a CNAME, which could limit your ability to use some anti-ddos/geo-loadbalancing solutions. Subdomain names (app.example.com) can be CNAME records.
I can also recommend: If you have any doubts whatsoever about the technical abilities of your descendants/family/executor, print the document and make sure multiple people know where the printed copy lives.
Land line is the safest bet.
In the US at least, there's also a guarantee that there will always be a dialtone on a paid line even if you have a power outage in your home (assuming of course that you have an actual traditional un-powered phone plugged into the line).
You could set up an "on app launch" automation shortcut for Maps to alter the Color Filter setting, but I'm not sure how you'd automate switching it back after closing/leaving Maps. I don't use the automations, but instead have a shortcuts widget with "toggle greyscreen" (among others) which is sufficient for me most of the time.
If the way companies like Google and Amazon act are considered, doing a chargeback through your bank is likely to get the subscription immediately canceled (and the accompanying account banned as well).