need alternative to touch screen attaching a wireless mouse to my iPhone 12 Pro Max to counter loss of touch sensitivity so I casn run my back up

If you're sure that nothing (cover, screen protector, dirt) is obstructing your touch screen, try a forced restart of your 12, using these gestures:

Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).

If the issue persists, it might be a hardware (battery? screen? other internal component?) glitch but we, just users like you, cannot diagnose that issue remotely, sorry.

I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.

Regards

Giulio

(*) You can do it:

• Online:--> Genius Bar Reservation and Apple Support Options - Apple;
• Contacting a local Apple Store or AASP among the list provided above;
• Using the "Apple Store" app, available and downloadable from AppStore.

i don’t think iPhone can use external input devices like keyboard or mouse. The most obvious choice would be a Bluetooth connectivity using Apple’s Magic Mouse but the compatibility notes don’t mention iPhones.

If you need a backup to iCloud I think you will have to rely on the automatic method which activates when the phone is on charger power and the screen is locked. Your only other option might be a usb hookup to a computer.