In 2017 there was a connector between ACC and ACS called PRIME.
I can't seem to find any content or HELP documentation on this connector.
HOW can we migrate or connect all our data and email templates into ACS form ACC? Is this connector package still available and if not can we at least get the older version of the package and tweak the code to at least give us a head start so we do not have to go through full implementation of ACS? There is an internal wiki mentioned below.
Adobe Campaign Prime: an offering based on the connection between Adobe Campaign Standard and Adobe Campaign v7 (AC7). AC7 is a new major version that updates AC6 with the ACSConnector, as well as a fresh new interface. Adobe Campaign Prime (ACP) extends the use cases covered by ACS by adding all the great features, channels and data capabilities from AC7 into one offering.
AC Prime paves the way for a smooth and progressive transition path. It brings all the power of AC7 combined with the prescriptive, cloud-based interface of ACS and its innovative campaign management features. Existing customers will progressively switch their digital marketing use cases to ACS while keeping the master marketing database, data connectors and sophisticated features in AC7. See Customer Transition Program in AC PMM Wiki.
As per my understanding and confirmed by Adobe consulting; Adobe doesn't support ACS Connector called PRIME anymore. Only companies which has procured the licence of this product earlier are allowed to use this connector but Adobe don't provide product guarantee anymore. You can get the official confirmation from Adobe Support/ your respective Adobe account manager.
Regarding your actual ask, if you want to share the data between both the systems you can use the custom solution by sharing the data between both the systems in the form of file upload(though its not the ideal solution). Hope this helps.