Funny, I just upgraded and don't have "nag screens" at all. After the update, I saw one screen explaining the new version, cleared it, and it's never come back, despite killing the app numerous times and restarting. There were also three alerts (the bell in the top-right hand corner) that explained a few things -- cleared those off as well. They haven't returned.
All of these things look like the default welcome to your new app explanations that just about every app has. They show up once, and are gone. Now it functions just like Fantastical2 for me, no nag screens at all. Also I just added a new event -- showed up on my Apple Watch instantly.
I don't plan on paying for a subscription, but as of now all of my functionality is there, no nags. We'll see how this plays out in the weeks ahead.