I used to follow subs like that fairly closely. I think most people loyal to ukr will stay on the platform that has continuity (reddit) because it is "life and death" and change could cost too much (credibility, missed crucial status communications, Russian hackers attacking a new and not yet hardened platform, etc.)
I think the user base that transferred over are mostly devs and techy people who saw the writing on the wall. A couple of the large instances have ~150k active users, so there may be almost 0.25-1million people here, but I have no idea how many are war information inclined.
The Ukrainian community on lemmy doesn't seem to be posting things that necessarily support the ukraine war propaganda, so perhaps a different sub would be required for bullet #2.
I hope someone else responds to correct me. I think that you creating the war community could be a great contribution if you have time to cultivate it!