Is there a way to add a title attribute to {{.TableOfContents}} links and internal links done by using [Text]({{<relref "some-content">}}) ?
Not sure, but check out https://gohugo.io/getting-started/configuration-markup/#markdown-render-hooks.
I have been there and as far as external links go it works. If i understand the docs correctly it is not yet possible to change anything but what is mentioned.
I also ventured into places strange and unknown, full of wonder and mystery and found this ("relref.html", {{ if len .Params | eq 2 }}{{ relref .Page (.Get 0) (.Get 1) }}{{ else }}{{ relref .Page (.Get 0) }}{{ end }}) but i don’t think i can use or modify it to do what i want.
It is a minor thing. Something that would be nice to have for the links. I am not sure if aside from cosmetics for the user there is any other benefit, like the SEO.