It is possible that a real invoice to be created before goods issue in ECC, and this real invoice to trigger the export customs declaration in SAP GTS, instead of the proforma invoice (the standard path)? Thank you
Yes, but the data-set for the Export Declaration might not be complete, depending on the situation in the feeder system.
If your "real invoice" is from an Outbound Delivery that has not yet been posted, then most of the required data should exist. But the despatched quantity and other data could still change prior to the Goods Issue, so that could be a problem.
If your billing document is created in another way, then of course we (forum contributors) could have no idea about the data-set or data quality, so results could be good or bad.
In the Customs world, it's best to keep the process in focus, so first think about whether your Customs Officer would permit you to declare in that way, rather than whether the system will produce results.