I think you are attempting this from the wrong angle.
All you really need to do is change your default shell.
From your current shell, what does:
echo $SHELL
show? I’m betting it shows zsh, which is the default kali shell.
Use the chsh command to change the default shell to Tilix. Something similar to:
chsh -s “path to tilix”
However, you will have to undo the changes you already made.