Buy EPM commercial module and use phones which are supported by EPM (afaik grandstream phones should be…) it is worth it for such many phones.
Or you can make some tools (grandstream has xml configuration file generator tool on their website) and generate phone config files pseudo-automatically. But I would recommend this only for few phones (about 10).
Remember that setting up the phones is just half of your installation. You also need the Bulk Handler to set up the extensions with all of the options you are going to need, plus you might need to import a lot of information into UCP.