Silencing all calls except my mum

You can certainly do this with Focus/Do Not Disturb.

• Go to Settings>Focus>Do Not Disturb.
• Tap on "People".
• Make sure "Allow Notifications From" is checked.
• Click the "Add People" and add your mum. Don't add anyone else.
• Under "Allow Calls From", choose "Allowed People Only".
• Whether or not you turn off "Allow Repeated Calls" is up to you. Doing so might enable someone else to get in touch with you in an emergency, for example your mum's neighbor or EMS calling on her behalf, though I hope it never comes to that.

You can turn DND on and off by going to the Control Center and tapping on the circular picture icon on the Focus button. To make things even easier, you may want to delete all of the other Focus options (e.g. Work, Driving). You can do that by tapping on each one, going to the bottom and tapping the delete button.