One of you (OP or the 24f looking for DMs) should make a community on LemmyNSFW (you are both members of that instance) for dirty chats.
I am on a different instance that federates with LemmyNSFW. I could not create the community without joining LemmyNSFW as well, and I have no interest in running that community. But, that is what you should do.
Or, since the two of you are both interested, assuming she favors whichever your gender is, maybe the two of you should just DM... but if you cannot find the place, make the place!