Boot to Single User Mode with Startup Manager
I am not sure how old your Mac is, but for Macs made in at least the last 10 years, this should work.

After booting to the Startup Manager, you can hold down the control key while selecting the operating system to boot. You should see the straight arrow change to a circular arrow. The selected operating system will now become the default.

Additionally, many Boot Camp arrangements allow the default boot operating system to be selected from the Windows Control Panel.