.bashrc is loaded on every interactive shell session, i.e. after login, so that would be too late. Also I did not see a way to took into this nicely, since this wait.service is a static one, i.e. cannot be disabled but is directly invoked by another already. Probably this helps:
mkdir /etc/systemd/system/plymouth-quit-wait.service.d
echo -e '[Unit]\nAfter=dietpi-postboot.service' > /etc/systemd/system/plymouth-quit-wait.service.d/delay.conf
systemctl daemon-reload