In a project I work on that used erlang.mk and rebar2 (for dependency and release building) I just replaced those two with mix and distillery. That required migration from .app.src to proper mix.exs and rel.config to distillery format, however I'm really glad with the improvements that the switch caused. The project is structured as umbrella, and has a lot of inter-project dependencies, and during the switch I found that some dependencies were missing in app files and with mix that was much more visible than in the previous environment.
Another benefit in such a switch was that I had to write a couple of compilers as our Erlang codebase depends on those:
The erlydtl compiler revealed a bug in a template, that was previously simply ignored by erlang.mk.
So now I'm really happy I can use all the tools Elixir environment provides in addition to the standard Erlang ones.