I got the feeling that having markdown as the “source of truth” was slightly easier for reading, checking, and diffing different versions.

Hmm. With groff, diffing is notably easier in my experience, as (unlike Markdown) you usually have one command per line and very little (or none at all) inline markup, whereas you’d have to count asterisks when diffing Markdown…

If you actually have a groff resume format that is very readable for you, I would like to take a look and maybe reevaluate if i’m doing groff the right way.

I actually do, but it was my first serious .roff file, so I don’t think it would be a good inspiration for you :)