I ended up with a hybrid approach where Hugo and Golang templates are only for building the data (TOML files) into the required JSON for a read-only REST API. The remainder of the solution is all JavaScript (in my case Vue and Nuxt instead of React and Next).
I came very close to switching to Gatsby early on, and I am very curious to know how my project[1] would have gone with Gatsby instead of Hugo.
I get the impression that for the data manipulation (avoiding complex Golang templates that generate HTML) and for build speed, Hugo takes it. Certainly if you are working in React the simplicity of a single framework is the way to go, as you did.
I also kind of felt a little more weird about mixing Gatsby and React with my preferred front-end framework, Vue, than I did about having to combine Golang template logic in one layer (API) with JavaScript in another layer (App).