I have added mani domain lets say “example.com” and I have to use partial CNAME setup. This domain has other subdomains which is used by other vendors and not needs to be on cloudflare.
If your goal is direct visitors from example.com to www.example.com you can easily accomplish that with the new Redirect rules. Example 1 in the Cheat Sheet covers that scenario. You will want to use a AAAA record set to 100:: to cover that setup without an origin. You can read more about that in the second link.
The easiest answer is move the domain to Cloudflare, it’s just DNS. There’s no need ro requirement to proxy anything you don’t want to through Cloudflare.
Absent that, the only way to ensure an IP address for the apex hasn’t changed is to either run a regular script to check for and update changes to the IP address in the primary DNS or to pay for an enterprise plan which includes an option for a fixed IP address.
The suggestion offered by @cscharff is likely to be the easiest. Having seen your concerns about child zone management in your earlier topic, there is no reason why you couldn’t delegate them elsewhere as needed.