Signal is great for one on one conversations or small groups. Matrix is better served for communities as federation offers censorship resitance, but they collect some metadata that Signal doesn't. They do have encrypted DMs, but it's less user friendly than Signal and if your partner is not tech savvy, I would go with Signal.
Signal has 2 points of concern:
Phone number requirement
Centralized server (one point of failure)
I use both for different groups of people. E.g. My Matrix account is publically listed on my Lemmy account, but my Signal one is only given to people I trust to some level.
Since all posts/comments are public, all you need is some kind of indexer to organize them and a list of all the instances. There are people working on it right now. But in the meantime you can just add (intext:"modlog" & "instances" & "docs" & "code" & "join lemmy") to your search queries to search all Lemmy instances.
It depends on the different clients as most of them started developemnt just weeks ago, but all most popular ones support moderating actions. But Lemmy lacks some hardcore/automatic moderation tools at the moment.
My advice would be to not wait for someone else to create communities would like to see, but create them yourself and just start posting. If you are not interested in moderating I'm sure you will be able to find someone else to take over.
You can post them in [email protected] to get some attention, but like minded people will find you.
In settings under sources, you can try changing the default target to load the full content if the images are not shown. It depends on the RSS implementation.
https://tedd.it/r/pics/comments/14rn8zd/the_rpics_moderators_cant_respond_to_reddit/