What's the 2020/Catalina method of getting my Mac to boot with Windows Bootcamp next time only?
One primary purpose of the &quot;nextonly&quot; option is to allow for installation of a startup manager. For example, if you install rEFInd as a boot manager, then you would configure the Mac to always boot to rEFInd. This boot manager can be configured to display different operating system to boot. After a selection is made, rEFInd instructs the firmware to boot this operating system once. After the selected operating shuts down or restarts, the firmware will boot back to rEFInd.
One possible solution to your problem would be to install rEFInd and configure to allows boot to macOS after a small delay. You could then during this delay choose to boot instead to Windows.
rEFInd has implemented a new feature that allows you press a number at startup. This number is stored in the keyboard buffer. When rEFInd starts up, the number is read from this buffer. If the number matches a menu item, then the operating system matching the item will boot. This feature was implemented to avoid having to make a select during the small delay mentioned in the previous paragraph.