If a service is mandatory or optional depends on your use case. For me is printing or sound not necessary, therefore I can disable cups and alsa service.
systemctl --type service --state running
to see what kind of services are running.
Stopping unnecessary services. In my case it was not enough to just run:
systemctl stop <ServiceName>.service && systemctl disable <ServiceName>.service
I had to mask them because some .socket are able to start services. Here are some services I disabled on my RPi without running in any issues:
sudo systemctl mask bluetooth.service
sudo systemctl mask cups.service
sudo systemctl mask hciuart.service
sudo systemctl mask alsa-state.service
sudo systemctl mask cups-browsed.service
If you set a fixed IP and use a LAN cable instead of wifi, you can also disable the following services:
sudo systemctl mask dhcpcd.service
sudo systemctl mask wpa_supplicant.service