Most people on reddit don’t even know about Lemmy lol. I’m sure a large chunk of users, particularly on lefty subs and programming ones, would love to check it out.
Ads definitely aren’t the way to go though since you’d be giving reddit money. Perhaps setting something up with mods of said subreddits?
Most reddit users aren’t on there everyday to see those. If you were talking about Lemmy on reddit for a month or two after things calmed down you’d see barely anyone there still know about this place.