Choosing between ConfureAwait(true) and ConfureAwait(false) should be considered when developing UI threaded application. In web project it is not relevant and can be ignored.
It can be a good idea to centralise user related endpoints in one place, and if having Identity Server in your infrastructure can be the right place for it. However, it can be not that trivial like it may seem. In this post I will describe the configuration needed for running local Api's on Identity Server.