Can I merge multiple Apple IDs into one?

No. but you can move data. This article can be useful for that:

Archive or make copies of the information you store in iCloud – Apple Support

You cannot move purchases from one account to another

But you can use family sharing to share some of the purchases. You need to have the old apple ID in a family sharing group.

Share App Store, iTunes Store, and Apple Books purchases with Family Sharing - Apple Support

Good luck!

Unfortunatley you cannot merge two Apple id's.