My first attempt at installing Debian via PXE had worked, but I wasn’t happy with it.
I tried it with udhcpd again, and it worked, after I removed some of the options from the udhcpd config file to leave it looking like:
interface eth0 start 10.20.30.50 end 10.20.30.80 opt subnet 255.255.255.0 siaddr 10.20.30.40 boot_file /pxelinux.0