Push notifications, icons, more themes, tons of powerful sort and save functions that weren’t on Reddit, Pixel Pals…probably more, but the last time I had Apollo Pro was many, many years ago.
There's a one time $20 fee to remove ads. There's also a thing for increased functionality that's separate and includes removing ads using a monthly or an annual subscription option, and there's also a lifetime option. I opted for the lifetime one at $100 because I figure I'll be using it for a few years, I'm happy to support the dev, and I really want all the bells and whistles.
At this point there are multiple updates per day during the beta period. I expect that will settle down once things stabilize, but for now it's definitely prudent to keep checking for updates.
There's a one time payment for ad free. The subscription is for ultra which is ad free plus a bunch of power user type shit that I don't personally have a use for.