Do I have to reboot between flashing ROM, root and Xposed? I am going to flash a custom ROM, systemless root (Magisk) and Xposed Framwork through TWRP.

Can I flash them all sequentially without rebooting, or do I have to reboot the phone between each installation?

My device is the Moto Z2 Play and my ROM is the Pixel Experience 8.1.
You can flash the custom Rom and Magisk without rebooting. However, I would reboot after flashing the custom Rom and Magisk. That way you can ensure your system and root are setup. Xposed is a bit hacking and it is best to flash on its own.

As far as Xposed you very important to reboot after flashing xposed. Also depending on Rom size it could take around 10 minutes to boot while Xposed does its thing.