automate startup disk selection with SIP enabled - see https://apple.stackexchange.com/questions/398711/how-can-i-script-the-bootup-disk-in-macos-catalina-without-disabling-sip

StartupDisk.applescript

do shell script "diskutil mount disk0s3"

tell application "System Events"
	tell application "System Preferences"
		set current pane to pane id "com.apple.preference.startupdisk"
		activate
		delay 1
	end tell
	
	tell application process "System Preferences"
		delay 0.3
		
		tell window "Startup Disk"
			set lockFound to false
			repeat with x from 1 to number of buttons
				if lockFound is false then
					if title of button x is "Click the lock to make changes." then
						click button x
						set lockFound to true
						repeat while title of button x is "Authenticating..."
							delay 1
						end repeat
					else if title of button x is "Click the lock to prevent further changes." then
						set lockFound to true
					end if
				else
					exit repeat
				end if
			end repeat
			
			set targetPrefix to "Windows"
			set targetIndex to 0
			set options to group of UI element 1 of scroll area 1
			
			repeat with i from 1 to number of options
				set x to item i of options
				if name of UI element 3 of x starts with targetPrefix then
					set targetIndex to i
					exit repeat
				end if
			end repeat
			
			if targetIndex is 0 then
				error "cant find " & targetPrefix & " in list of options"
			end if
			
			key code 48
			delay 0.1
			
			repeat with i from 1 to number of options
				key code 123
				delay 0.1
			end repeat
			
			repeat with i from 2 to targetIndex
				key code 124
				delay 0.1
			end repeat
			
			click button "Restart…"
			delay 0.3
			click button "Restart" of sheet 1
			return true
		end tell
	end tell
end tell

needs "Use keyboard navigation to move focus between controls" to be enabled, see https://apple.stackexchange.com/a/422191/152562