The advice to confirm you keep the logins and backups if you cancel a plan is the one small business owners most often learn too late. We have taken over sites where the domain, the hosting and the only backup all sat in an account belonging to a previous agency.
Two other points from your list match what we see. A backup that has never been restored is only a hope, so a test restore every few months is worth more than more frequent snapshots. And renewals fail because the card on file expired, not because anyone forgot the domain exists.
On client work we hand over maintenance as a short written list: each job, who owns it, how often, and what proof looks like. For example, the contact form is submitted on the first Monday of the month and the email is checked in the real inbox. Owners can then see that the work is happening.
Do you find owners prefer a monthly report, or only hearing about problems?