I follow a sub that’s all reposts from reddit. Occasionally I think about replying to something, but then I just go, “What’s the point? OP isn’t here.” I don’t recall ever seeing anyone else respond to any of the crossposts, either. The community is c/[email protected] if anyone is curious, which is a pretty niche topic to start with.
I’m not convinced it’s adding anything to the Lemmy experience, but at least those are clearly marked as crossposts and are all posted by one account, so it’s easy enough to ignore if I wanted.
On the “all” thing - remember that reddit has a mode, which is the default, that’s between Lemmy’s “truly, everything all” and “subscribed”. In this mode, you’d get popular posts on subs that had opted in to allowing them to hit that page (or didn’t opt out, I don’t remember).
/r/hockey is a good example - their posts usually generally stayed in the sub, but their Super Bowl post (and occasionally others) would usually hit reddit’s front page and bring in a ton of people who weren’t subbed to /r/hockey.
This was a good feature of reddit, I hope Lemmy eventually gains something similar.
It’s possible I misunderstood your last goal, but if you’re planning to have Lemmy comments posted back to reddit, I suspect that wouldn’t go over well with reddit’s admins after they figure it out.