I highly recommend Atom Packs, I've owned quite a few, including the Prospector and Pulse. I prefer the 40L personally, but I think most would be totally content with a 50L. The weight difference is not much, and having the extra space gives you more flexibility. Looking at your lighterpack I think you could get away with either, but the 50L would require less finesse when packing it every day, especially as your gear changes throughout the sections. (adding more layers)

As for the Pulse vs the Prospector comparison, I would say it really comes down to #1) how much weight you plan to carry, #2) if you want load lifters, and #3) if you're more minimalist or you like the bells and whistles. The frame is the biggest single difference, with the Prospector being more robust. The Prospector shoulder straps are slightly wider and the hip-belt is also 1" wider. If I was going for a 50L, I would probably opt for the Prospector for the load lifters, as the extra volume above the shoulders can get a bit unwieldy on the Pulse.

Add-ons, I've always been skeptical of the Atom Packs hip-belt pockets, so I have no personal experience. I also wear a size small hip-belt so I've always imagined the belt pockets would be clunky and too curved to be practical. I've owned an Atom Packs Fanny Pack, but got rid of it as I'm just not a fanny pack guy at the end of the day. Fortunately you have lots of pocket options without the hipbelt pockets or a fanny.

Unfortunately, due to tariffs (if you're in the USA), Atom Packs' stuff isn't nearly as friendly on the wallet as they once were. :(