Here maybe some things that others may have forgotten or may not have emphasized as such:
Continue supporting the software development
I'm really surprised how well developed and rich of features QPixel is, even more after hearing that it is an effort of only two main developers and a few other contributors. The goal should be to make it the best open source Q&A software available with the best features. Codidact can serve as the showcase for QPixel but will also profit from a better QPixel. And who knows, QPixel can even be competition to something like to Stack Overflow for Teams, if only there was paid support for QPixel, which might happen at some point.
Favor quality over quantity
What is more useful? Lots of mediocre questions and answers or fewer but better quality content pieces? I would opt for the letter, because Q&A is not new and exists in many places. High quality Q&A is comparably scarcer and it might be easier to make a high impact there.
What does that mean actually? It means that we should clearly formulate the expectations on the quality of a question and an answer. It also means that editing and improving existing content is something really worthwhile and should be encouraged. Showing the number of edits on the user card is actually a step in the right direction. We should have a culture of improving existing content.
We must be nice
With small communities you cannot afford to lose even only a single contributing member. While being nice and polite is always a good idea, I think here it should be paramount. Not only would it reduce attrition, small communities can also increase their attractivity a lot by how welcoming they appear.
Example: Negatively received (more downvotes than upvotes) contributions may not be very useful but I would not call them "drivel" because that is a too strong/too negative label that does not serve any use. Better might be to think of negatively scored content as content that yet needs to be improved or simply wasn't a big hit.
Talk about Codidact and QPixel
There are actually two topics one can talk about (but always also mention the other one while doing so): the software and the Q&A platform. How do they compare with competitors, what can they do better, what do they still need, ... Not only on Social Media but also on all "journalistic" platforms. For example, submit an article to opensource.com and talk about it all. (If needed I could probably also do it, but I don't know the history of Codidact as well as others.)
Don't put inactive communities in the freezer
What good will it do to put an already suffering entity in the cold? None! I don't really like the division into sub-communities with the sub-sites much. It has advantages but also disadvantages. If possible, I would rather try to blur the separation between them more. A network user profile in the software would be one step towards that.
If we see the site as a whole we see places that are less crowded, but the knowledge is still in there and can be (software is open source, knowledge is under a free license) be restarted at any time. I would not put sites in the freezer. Rather keep them and wait for new activity. More like hibernation actually and waiting for a new spring. It might be a good idea to mark these sites as such: "hibernating" or "currently low activity".
The main issue was that ghost towns do not make a good impressions. But I think if they were marked as such (say a banner like
This site has had low activity in the last time. You can help revive it by ...
should be sufficient) and on the codidact.com landing page maybe also a visual indicator of activity and sort by activity. But please do not put them in the freezer, that way you will likely lose them and have to start at square one again.
Face the realities, 2021 is not 2009, open source is not venture capital
There is already a lot of Q&A out there and the marketing reach of Codidact is limited. It looks to me more like a grassroots movement that will grow mostly by word of mouth, so probably not very explosive. What sold me to join was actually the quality of the software (really impressed, makes we want to be able to program in Ruby) and the quality of the content, not so much the amount of content that is existing. Codidact will probably only grow slowly.
There might even be a critical mass required for taking off and that might never be reached. The fun is trying to beat the odds though.
Add existing high quality content but increase the quality even more
I read about mass imports and that search engines basically did not honor that and ignored the copied content. So pure copying probably isn't useful, but copying and improving the quality content (more concise, leave out outdated information) would give a real added value and hopefully search engines would at some point realize that too.
That's what I will do if I have time, because that's (editing and summarizing and improving) is what I can do well and it doesn't depend on the size of the community really.
Everyone, think what you can do and simply do it
Improve the software. Ask or answer. Spread the word. Everything will help.