RES as an extension that is separate and distinct from reddit itself is necessary because the community has no way of improving the reddit web-ui directly. I'd hope that anyone considering working on an enhancement suite extension for Lemmy would also consider working with the Lemmy devs directly to just add the stuff they want into Lemmy, either as default features or options that can be enabled per-user. Ideally I think we'd want the Lemmy web-ui to be good on it's own, not require a browser extension to suck less.
As another commenter noted, there's already a GitHub issue for some of the keyboard shortcut stuff you're interested in.