Closely related to your proposal (as backup files can also be used to add second devices) I suggested here letting people make lightweight backups that do not include chat histories and files, which will make the process much faster and also not use so much storage on the backup media:
I think it makes sense to let people choose to limit the scope of data sync and also to limit the scope of backups and I suspect that these are two sides of the same coin which could be addressed together.