I have three tips for you. None of them is perfect, but maybe you find a worthy combination.
Grammarly. The problem here is, that they hide the real good tips behind a paywall, but the free version finds plenty of issues. You will probably have to use their website to write your text (or copy-paste it), which is annoying. They have browser plugins, so if you find a way to use forestry.io or Netlify CMS in your daily workflow then that might help… I used it right now on this last paragraph.
Typos. This is a program that checks your MD files for errors. I have it in a setup together with pre-commit. This is a pure programmers solution because you have plenty of setup and configuration to do before it works nicely. Typos also is a bit blind to computing specific vocabulary and you might have to create a huge whitelist. The advantage is that it works over all types of files, not just Markdown.
Spell Right. This is a VSCode plugin. You might assume that this is what you want, but in my usage, I experienced a very slow VSCode. So I am enabling it only while working on copy.
There is another spell check plugin for VSCode, named Code Spell Checker but this one sends your writings to a server and analyses them there. It’s also more for code and less for “real” writing. But you might want to have a look.
None of these solutions is perfect. On Linux, you have very lightweight dictionaries that you could use in a bash script to check your files, but I never tried doing that by myself, so I guess that is not a real option.
There is no way to integrate any spell checking into Hugo directly, so it’s always something that runs on commit (if you use Git) or in the environment I am working with (VSCode, Browser).
There seems to be an unofficial vscode plugin for grammarly, but I haven’t used it.
There is also a solution called LanguageTool (https://languagetool.org) this used to be free, but it seems they’ve also added a commercial part so I’m not sure of their status.
Currently I use Code Spell Checker, but that’s only for spellchecking, it doesn’t catch grammar errors.
I write on the forums containing the audience for the writing. So if I’m writing something for Hugo, I might post a draft here, get my feedback, and then publish to a canonical archive.
As there are forums for just about anything, I’m able to consistently passively collaborate with folks online.