Is there a way to boot FreeBSD over TFTP?
All the instructions I've seen require you to transition to NFS after
PXEBOOT has loaded.
No, you can only grab a kernel image and initial ramdisk over TFTP.
Once the kernel has loaded it needs somewhere to load the root filesystem from. The FreeBSD kernel only knows how to use UFS, ZFS or NFS as its root filesystem.
You may be able to create a ramdisk loaded over TFTP from something like mfsBSD to have a UFS image in memory.