The beauty of Lemmy being open source is that an RES-equivalent shouldn't be necessary. RES existed because there were so many qol improvements that the admins were unwilling to implement. But here, you could always just make a PR and contribute the improvement into the codebase
Developing some sort of a settings UI would probably be a good starting point. Kbin already has the sidebar settings so that is a logical place to put any enhancements, but Lemmy settings are not quite as accessible.
I wanted to do that, and I started on an extension for it, but as others have said, lemmy being open source means that improvements should mostly be made on the website itself, rather than on a browser addon.
The downside is that for user settings, they are stored on the instance's database, which means that you need to know a little bit of rust to implement a new setting (I'm doing that right now to open comments in a new tab instead of having to middle/ctrl click)
That's true. The only caveat I see is that if the Lemmy devs don't agree with a change it will never make it into the code, but with Userscripts the sky is the limit.
Although I think it's wise to make a PR to Lemmy first.