Lemmy, too, has algorithms that determine what you see - how many upvotes a post has, how many comments, how recent, etc. The communities you subscribe to may have some high-quality, niche posts that you’re very likely to miss because they’re overshadowed by bigger, more active communities where posts simply gain more traction - RSS lets you circumvent that.
i3 isn’t a proper DE, though, but I definitely would go with that with that little RAM.
For strictly DEs, I’d pick XFCE - it’s just lovely for what it demands.