Hi, I'm one of the developers of Invoice Ninja. Really cool to see all of the interest in v5, we've been working on it for a while now and are extremely excited to release it!
We actually just reached out to Clourdon to discuss getting the package ready for our stable release in a few weeks, they shared this thread with us.
If you have any questions about the new version I'm happy to answer them. Also, any feedback on the new version would be very much appreciated!