The problem could be much smaller, if after the installation deltachat would collect the known addresses, more precisely the “email-approved” addresses, from the preexisting messages on the server. (And completely syncing the address book could then only be a matter of placing a list of minimal “messages” into some /DeltaChat/contacts subfolder.)
For a further description look for MsgSCAN in [Wiki] IMAP strategy to chat within the email ecosystem .