Agree there was a shift, and while it hadn’t occurred to me before, your timing makes sense.
Reddit used to be that place where you had definitely “read it” first.
Now, to your point, most subs that bubble via algo to all and popular are tumblogs (infinite image/meme scrollers), and posts come after it’s already been on IG and Facebook groups.
I’d chalked this up to IG influence, but Tumblr diaspora makes as much sense, together with your hypothesized shift to pop engagement over niche quality.
Still works that if you dump the subs that show up in all or popular, you can get back to text / long-text niche interests. Most seem to be slowing pace of posts and discussion replies though, so unclear how much longer that will last.