I’m running into this with my work hugo migration and I’ve actually had good feedback from using the gitlab webui to edit files in-place. Most users just need to edit content markdown files so they can do this quite easily through that interface and never need to know the ins-and-outs of git to do it.
You could even theoretically make the hugo /content/ directory a separate repository so they can only edit content files and not the site config or layouts. And now that I’m saying that out loud I think it might make sense in some cases to explore that.
Another option is to integrate a CMS front-end like https://www.netlifycms.org/ - we actually had that setup for the main RawTherapee website once upon a time to test it and it seemed to work quite well. The benefit there is the WYSIWYG preview of the markdown along with an editorial review process baked in (draft, awaiting approval, published, etc).
I’m actually starting to look at things like https://fastapi.tiangolo.com/ to produce API endpoints for any data submissions that need to happen and to integrate it with my static site for low-effort tasks (filling in a form or something similar that just needs to do something with the data that’s simple). This way the form submit just hits a predefined API endpoint that does one thing mainly. 
My old company was sharepoint as well and it’s just a mess of tech stacks all smooshed together in weird and convoluted ways I thought…
