If you are struggling with CHANGING the boot disk when you are booted to Windows, then I suggest that you do not change the selected boot disk. Boot to your macOS system. Open Startup Disk (in your System Preferences), and make sure that your macOS boot disk is selected.
Go through the Option-boot screen to boot to your Windows system. The macOS boot system remains selected, because you don't change that setting. When you are done using Windows, simply restart. Your macOS system is still selected, so it will boot to that by default.
This will work, because your boot disk settings don't change.
(Your Time Machine backup is also bootable, so if that contributes to your issue, just disconnect the Time Machine drive when you are using Windows. Time Machine doesn't backup anything unless you are actually booted to your macOS system. Reconnect the backup drive when you boot back to your macOS system.