For my project, the lack of a document merge output to an immediately editable form was one of my biggest obstacle, so I manage to create a rudimentary document merge functionality.
An action button, copies/posts data to DocTemplateForm in a separate Google Sheet & Table. Example, for a letter, this data includes name, address, project/file no, File Name, a destination GDrive FolderID, and a Google Drive ItemID of a pre-built customized GDrive Doc Template which has the Column Names of the sheet data to be merged, etc. When this sheet’s script is triggered, it copies the specified template Letterhead to the specified GDrive Folder ID and then performs a search/replace with the relevant data posted to the Document Template GSheet. Best of all the script then places the new document’s Google Drive ItemID into the original “document table”, so that the UX will replace the “Merge Doc” button with an “Edit Document” button. (This does require an extra refresh to update UX)
I’m currently using an action button to “go to another view” to create the merge table entry, but I intend to streamline with the new “add row to a diff table”.
It’s clunky, a bit slow, but it does work.