Is there a way to enroll a device without activating first?

If you purchase through an authorized reseller that supports automated device enrollment (enrollment into Apple Business Manager), you can enroll it that way. This does not require the device to be on at all. When the user powers on the device, they can begin to set things up, and it will force the device into management when Apple Business Manager is tied to your MDM.

tobias209 wrote:

If I want to prepare a phone before deactivating the user's old phone.

no Transfer data from your previous iOS or iPadOS device to your new iPhone or iPad - Apple Support