I watched the first few seasons of Arrow, then stopped with about 5 episodes or so left in season 6 and only watched the Elseworlds episode and the last 5 minutes of the season finale in season 7. Season 8 for the most part did not require much knowledge of events in season 7. I could pick up most of what was needed easily enough. What I had seen in previous seasons was definitely good enough. I think the first episode this season of Arrow is the most important for Crisis - Arrow is on a mission to get something from Earth-2. I am glad I watched the episodes this season.
I have yet to watch any Batwoman and only watched the first episode of Supergirl so far. I may catch up later, but it is not a priority.
I am up to date on The Flash and am looking forward to seeing what they do with Crisis. I think there was absolutely no reason to show the Nash Wells scene at the end of every CW show. Except for the Bloodwork monster form being a bit over the top, I thought this season of The Flash has been decent. I especially liked that the first arc has wrapped up.
Black Lightning is consistently the best written of the CW shows. From the preview it definitely seems that the next episode sets up his appearance in the crossover.