You have a point here, not sure what would be the best way to handle this?
On the one hand the sticky thread keeps things neat and organized here instead of having everyone post a photo, on the other hand as you said it is new threads that appear on your "subcribed" or "local" home feed, not new comments (unless you choose to), so photo comments in the megathread will have less visibility (except for those who specifically come to c/googlepixel, as they'll see a "(XX new)" in front of the comment count of the sticky thread if you use the web UI).
A weekly thread would probably have the best of both worlds but I'm not sure if we have enough activity to sustain weekly photography threads.