!Ukraine@sopuli.xyz is probably a good place to post, it seems where most of the news on the war aggregates here, but I am not a mod or the instance admin, so take it as hearsay for now.
As to how, I don't use Mastodon myself so I am unsure, but I think I heard/read people on here talking about tagging Mastodon posts in a way they appear in certain lemmy communities, even as comments to threads (that would make it possible to gather them in dedicated threads instead of filling the community with posts, too). Sadly, I can't provide how to do that, but maybe someone from Mastodon can help with the how-to there.