OK, so I took another look at it and now I remember why I did not implement the choose destination dialog in the first place. The reason was that it is only possible to move an existing file to another destination in the cloud or on the iPad. It is not possible to write files directly to that destination. This means that you always needed enough free space in the ST3 folder to create the backup even if you wanted to write it into iCloud. So I did not see an advantage of having this dialog as in my opinion the Files app is better to use for the purpose of moving files around.
But I see the confusion some users have when creating backups and not knowing where to find them. So I will add this dialog in version 3.4.