Creating a Central Store in the existing domain

Hi all, hope someone could point me into the right direction. We have a single domain with 10 sites, 15 domain controllers ranging from 2008 r2 to 2016 and about 15 GPOs. The master is 2008 r2 and I believe that all GPOs were created on this master. We are looking to create a central store and also migrate the master to 2019. 

My question is after creating a PolicyDefinitions folder inside the SYSVOL folder should I be copying all ADMX/ADML templates into that folder from the PolicyDefinitons folder on the 2008 r2 master and then adding newer templates into the central store when the need arises? 

Or should we migrate the master to 2019 first, decommission the old master, create central store folder and copying ADMX/ADML templates from Policy Definitions folder on the new 2019 master? 

What might be some possible repercussions of creating the central store having already number of active GPOs in the domain?

Thanks!