So let me just clarify again and its just so that you understand and when you explain to people it will be clear.
You are not moving data to an App. You are asking to move data from an OnPremise D365 to a Dataverse Instance in the Cloud. What I do not know is if that is pure Dataverse or if its D365 in the cloud.
I ask because of licensing. If the data itself is from a Licensed D365 Sales./CE etc on premise then you cannot simply copy the data to Dataverse, even in the cloud as its not guaranteed, license wise to cover that data. I will let you worry about that with Microsoft, but I want to warn you up front as the data is... licensed to your D365 instance and I know it sounds silly but you do not own the data in a way that just lets you put it anywhere so please be careful that you have licenses to the data covered properly is all.
As for migrating the data, that part is not hard, you can even use tooling in XRMToolkit to copy the data.
If these suggestions help resolve your issue, Please consider Marking the answer as such and also maybe a like.
Thank you!
Sincerely, Michael Gernaey