How can I stop iCloud from asking me for my iCloud password on my iMac?

As you mentioned - iCloud asks me for my password which is the same password as my iMac.

Admin password to sign with Mac , should never be same as Apple ID password .

You can change Apple ID password Change your Apple ID password - Apple Support

Read this article Tips for creating secure passwords on Mac - Apple Support

When you sign in with www.icloud.com from a web browser .

It will prompt to enter admin password , after entering it the iCloud account is accessed , it's quite normal , as the trusted device ( the iPhone ) that is verified in the account is not synced ( signed in with same Apple ID and password on the same network ) so Mac acts as a trusted device and its located using IP address , the find my Mac is turned as on , so it prompts to enter admin password , it verifies that the right administrator is trying to enter the account , no other person can sign in as only administrator knew login password , the account is set with 2FA .

Note : make admin password as in from of some English word ( at least eight characters ) , rather than alfa numeric code ( that is four digit ) see https://support.apple.com/en-in/guide/mac-help/mchlp1550/mac