How to set a default email address in Apple Mail

I am running my Silicon iMac on Sonoma 14.4.1. and using Apple Mail for my two email accounts. The main account is iCloud and the other one is Google.

When I open Mail to compose an email I only get the option to either use Google or Hide My Mail. No option appears to use iCloud.

In settings I went to the setting for ‘Send new messages from.’ This showed ‘Automatically select the best account’. The only other option was Google. No option for iCloud.

In settings I went to Accounts/Account information and found that the Google account showed the full Google address in the Email Address entry but the iCloud entry for that section was blank.

I added the full email address for the iCloud account so that it appeared in Email Address and Mail ran correctly giving me options for either account when composing a new email BUT after restarting the iMac the Email Address section in Account Information was blank again and no options appeared in Mail for using the iCloud Account.

Is there any way to set up my iCloud address as the default or ensure that my iCloud address remains in the Email Address section of settings?

I rarely use the Google Account and it is a real pain having to set up Mail for iCloud every time.