Hello, I'm fairly new to FreeBSD, so my thoughts might be pedestrian or obvious to most... but when I've had troubles like that, my two solutions were:
1: It defaulted to the Wayland session in lightdm (or whatever display manager you use), and I had set up Xorg, so I just had to select X11 session when logging in.
2: I've been silly and have had typos on pretty much every step of Section 8.2.3-XFCE of the Handbook, particularly the sysrc commands, which then adds the typo to the /etc/rc.conf but it doesn't have a verification check if it makes sense... e.g.
# sysrc dvus_enable="YES" will ACTUALLY apend "dvus_enable" into rc.conf instead of the intended dbus_enable.
Hopefully you get some luck and have some success!