It's official for next year.
With regards to the idea of the one big bad for each show, obviously it would be difficult with Supergirl taking place on a different Earth. However, I'm almost wondering if the best way to do Crisis would be to do a long-ass crossover. I'm talking like 4 or 5 episodes of Flash and Arrow. Or better yet, announce Crisis on Infinite Earths as it's own program, getting a 13 episode series in Fall 2019, and the regular Arrowverse shows get half seasons picked up in January and start with the fallout of Crisis. Doing that will keep it to one episode a week, which may keep from overwhelming people with the same characters 3 or 4 times a week.