There's activity on all of them. It would be helpful to keep all the discussion in one place. Have these communities (and others I am missing) discussed any plans to push the discussion into one community so we can gain economies of scale in sharing/discussing news related to Ukraine and Russia's immoral war against the people of Ukraine?
Nope. The Fediverse is built around this kind of spreading out, it's in the fundamental design of the platform, and a sign it is functioning as intended.
There's probably some third party software by now that you can use to customize your own feed, and bundle them all together if you want. I'm not sure what'd it'd be though. I just use a couple of the communities, personally.
Reading across instances misses a lot of posts and comments.
Why, though? I thought the whole point of this federation thing was to distribute the content between instances. I would expect that to be all content, not just some.