How can I merge iCloud and Store Accounts

If you decide to stop using one account, stop using your iCloud account because all purchases made using the Store Account ID are tied to that ID.

There may be some data loss, but you can minimize it. If syncing photos with iCloud, ensure Optimization is turned off in Settings/Your Name/iCloud/Photos. Then make sure full resolution photos are on the device. When signing out of iCloud, if asked whether you want to retain data, choose yes. Messages conversations will be lost if they are addressed to the ID. Also see Winston Churchill's response in the below discussion

How to transfer all files from one iCloud account to another

If you have 2 different Apple IDs, one for iTunes, and one for iCloud, you cannot merge them.

If you are using the same Apple ID, to log in to iCloud and the iTunes & App Stores, then they are already one and the same.

Otherwise, you would need to stop using one account, and just use the other.

Edit: Sorry, I read "manage" not "merge".