How do I move all my iCloud to another Apple ID?

Merging Apple accounts (IDs) or transferring data is not officially supported, nor can it be done easily or completely.

Apr. 2018 post by ASC helper Winston Churchill on transferring data from one Apple ID to another. https://discussions.apple.com/message/33333380#message33333380

Feb. 2021 tips from Eric Root https://discussions.apple.com/thread/252468781?answerId=254653802022#254653802022

Look into using Family Sharing to continue to be able to use items purchased with one Apple ID with other Apple IDs.

Family Sharing - https://support.apple.com/HT201060

Set up Family Sharing - https://support.apple.com/HT201088

For files, photos, contacts, calendar, notes you'll need to do so manually by copying each item to a computer or device and then adding it to the new account in the respective app again manually. Backups cannot be moved.

There is no automatic process. Unless there's an exceptionally good reason to change accounts, it should not really be done, and its better to keep the account updated and accessible.