Thanks for doing due diligence in the docs and community- we really appreciate that. You’ve probably seen this already, but if not, you may find some additional “gotchas” in there so it’s worth checking out:
One question is: are you able to build this locally? If so, then I think we can start looking at mismatches with the Netlify build image aside from Hugo, like with npm/yarn, etc. But that’ll be important to know first. If it’s not building locally, there may be incompatibilities between new dependencies and your 0.4x Hugo version (and/or your Mainroad theme).
Let us know what you find!