A simple script update CloudFlare DDNS for Synology NAS. This script can be integrated into Synology NAS UI. It largely refers to official CloudFlare's API example for Python

Installation

Install python3 from synology package using the NAS web interface.
Enable the SSH connection and ssh into your NAS
Install pip for python3, then get python-cloudflare

sudo python3 -m ensurepip
sudo python3 -m pip install --upgrade pip
sudo python3 -m pip install python-cloudflare

Download the CloudFlareDDNS script from this repository. I personnaly prefer putting the script in /usr/local/bin/. You can put whereever you want. Just remember the path.

wget https://raw.githubusercontent.com/lioujheyu/synocfddns/master/cloudflareDDNS.py
sudo mv cloudflareDDNS.py /usr/local/bin
sudo chmod +x /usr/local/bin/cloudflareDDNS.py

(optional) You can firstly test the script's functionality. Running this script without any arguments gives the usage, like cloudflareDDNS.py <username> <api_key> <hostname> <ip_address>
username is your CloudFlare username, usually the email address you registered in CloudFlare. api_key is your personal CloudFlare API key. See here in how to retrive the key.
Integrate the script into Synology DDNS management interface by adding the following text into /etc.defaults/ddns_provider.conf

[Cloudflare]
        modulepath=/usr/local/bin/cloudflareDDNS.py
        queryurl=https://www.cloudflare.com/

Go to DDNS management page in your NAS web UI (control->external access->DDNS). Click Add. And select Cloudflare from the drop-down menu. Fill the three necessary fields which are hostname, username, and password(CloudFlare API Key).

That's it. See if the DDNS' IP has been updated in your Cloudflare page.