scheduling full sync by DSA

The situation is most difficult. As I told, it takes time to describe the whole configuration. There is another option, which could solve the timing problem in our case. If we use different EA in source for FIM and DirSync, we could create target object for matching before we move source object to the sync. For example EA8 for FIM and EA9 for DSA. Source Admin has to set first EA8. Then two hours later set EA9 and later set EA13.

Just a little bit more about the configuration in our environment:
I agree with the idea to get GAL full before migration of the first mailbox. Mailbox enabled users in target will not work, because we have to migrate 8 Exchange organization parallel. And each of 8 has more than one Exchange Server in up to 5,6, or 7 in other trees, which we are not allowed to connect to their Domain Controllers in first migration step. Instead of one by one Exchange org. we have to migrate only Mailboxes on first Exchange Server in each 8 Exchange Organization. Unresolved links are daily business, but this is another problem.

If we use different EA in source for FIM and DirSync, we could create target object for matching before we move source object to the sync. For example EA8 for FIM and EA9 for DSA. Source Admin has to set first EA8. Then two hours later set EA9 and later set EA13."

That will handle the matching issue on the first pass, when EA8 is written. But that still leaves the mailbox enabling. That requires an attribute that is controlled by MBRedir to be updated. That is where the proxyAddresses update comes in. 

Just a little bit more about the configuration in our environment:
I agree with the idea to get GAL full before migration of the first mailbox.

It is not just an idea, it is the standard process for the migration tool. You can't grant permission to a resource you are migrating to an object that is not yet present in the target. So Delegation fails to migrate. This is a much bigger deal if you are migrating public folders. With just mailboxes, close sets of working groups can be enough, but there will still be some fall out. 

Mailbox enabled users in target will not work, because we have to migrate 8 Exchange organization parallel.

The full GAL is is tied to each Exchange org's migration, not all org. So when it starts to be migrated, the GAL for the org should be complete. And by complete, I mean complete with all in scope objects for that migration "pair" or as you said "one by one".

IHMO, FIM being as flexible as it is yet when a migration is in the mix, most FIM owners are reluctant to make changes to support the business needs. 

On a Side note: Migration Manager for Exchange supports third party sync engines, I.E. FIM. This means you would let FIM do it all. MMEX would migrate the mailboxes and FIM handles the directory. This includes proper email addressing and switching. This might have been an option here??

"This includes proper email addressing and switching."
That means to skip all attributes for DSA? There are some one, which are mandatory like attribute 'proxyaddresses' which we could only skip by manipulating of skipattribute.xml file, which is by the way not supported but it did help us. Please check 'SR Number:4146391| - Skipping of IMAP and POP3 status form source to target'

"This might have been an option here??"
For this we need a list of which attributes have to be skipped by DSA.