Hi squirtbrnr ,
I updated the upods.net guide that I wrote to select "No Mapping" on the Synology. After changing that setting on my Synology, I noticed that I was still getting a "connection refused" error. After checking the log, I saw that I just needed to create a folder at /storage/.cache/ssh on the TFTP server and then I was able to establish an SSH connection. I am also using a Raspberry Pi 3 and have the same issue with booting. I found that by copying a newer version of bootcode.bin to a blank micro SD card fixes the problem, but then an SD card is still having to be used which kinda defeats the purpose of network booting.