Minitest (which is bundled with Ruby these days) is Good Enough™. It doesn't require you to learn a new DSL. Everything is just Plain Old Ruby. The use of RSpec these days—IMO—is just cargo-culting forward what was the right decision from 10+ years ago. Having an pseudo-English style interface for testing isn't worth having the additional dependencies nor the mental overhead of needing to know how the RSpec syntax is actually mapped into Ruby concepts.
I'm not asking you to change it, you're welcome to have a different opinion than mine. But I am curious if you have strong reasons for requiring it. Particularly because MiniTest seems to be well-aligned with the rest of your design philosophies, unlike RSpec.