How do I merge Apple IDs and transfer data between accounts?

Appreciate this question has been asked but asking again in case there's been some resolution in recent years....

Before Apple developed me/icloud accounts your Apple ID was an email address, such as gmail / Yahoo etc.

Then me came along and you could then set up a me (now iCloud) email address.

So somehow I have iMacs and MacBooks on my old pre-me address but my phone/pad works off my me address, and there's no apparent pattern as to which apps use which ID.

So...is there a way of merging the accounts or if not an easy way to transfer data out of one account onto a third party, and then transfer it to the other. Basically I am fed up with this and would like to have one ID with all apps and data on it.

TIA.

[Re-Titled by Moderator]