The JS-API provides the 'plumbing' to the REST APIs which will allow you to create the UI components for managing users, groups and permissions. The ADF roadmap does include managing permissions via pre-build UI components, you'll see this work starting to appear from ADF version 2.3.
There is an Open Source project, released at the end of last year, which is not designed to replace Share but is to demonstrate how to create a file/document app using the ADF, a sub-set of the Alfresco Share features. You can get code from here: GitHub - Alfresco/alfresco-content-app and even contribute to the project to help us progress it faster. Version 1.1 of this project is expected mid-March.
Yes it is all possible if you want to put some time in it. I have written a dashlet components and I have developed my own dashlets, but they are written in Bootstrap (not Material like ADF uses), so I don't know if they will be any use to you. I also created user lists. It's not so much work if you use the ADF services and know how Angular works.