I seem to have found myself a reasonable solution somewhere but cannot remember where i found it.
tested on 8.1
Place at the top of .zshrc
export PATH=/usr/bin:/boot/dietpi:/bin:/usr/sbin:/sbin:$PATH
Place at the bottom of .zshrc
/boot/dietpi/dietpi-login
Seems to have restored the majority of functionality for me, things like dietpi-launcher commands and the login motd.