For document scanning I use this https://f-droid.org/en/packages/org.fairscan.app/ directly on the phone, works well even with a shitty camera.

I never had the need to print from android but I found this https://f-droid.org/en/packages/io.github.benoitduffez.cupsprint/, it might work.

Scanservejs looks nice but one more service to maintain and secure. I guess I’m a minimalist.

I use SANE for scanning.

• install sane-utils
• configure /etc/sane.d/saned.conf, list the IP adresses or network allowed to connect
• start saned.socket

on your client (linux desktop)

• add the SANE server IP to to the config echo "192.168.xx.xx" > /etc/sane.d/net.conf
• list the scanners scanimage -L

The scanner is also automatically listed in scanning utilities like simple-scan etc. Don’t know about phones or other devices, I assume there are scanner auto-discovery utilities; phones also have cameras so thy can scan on their own. I don’t know about Web UIs, why not just use native printing/scanning features of guest devices?