Congrats on the new tank! The Reefer-S 850 is a solid choice - it's the same tank I've been eyeballing for my next upgrade. I love that they moved to aluminum stands for the "S" series as the standard marine plywood Reefer stands are heavy & clunky.
The extension sump is so, so cool. I think it would make for a great frag tank since you could run it on a reverse lighting schedule to balance pH during the night but not have to worry about the light spilling into the rest of the sump and growing algae. Could also make for a great chaeto/refugium setup.
I have a Clarisea that I plumbed into my Reefer 425 XL sump and I love, love, love my Clarisea (helps keep nutrients down, less detritus to siphon, water is incredibly polished), but it was a pain to plumb. The adapter that comes with the ReefMat looks like a lifesaver. The Clarisea plumbing pieces were not standard so it was difficult to get it hooked up to the Red Sea plumbing + I had to razor out the Reefer sump default filter holders (not sure how the ReefMat will get around that). The ReefMat definitely has more features than the Clarisea and if it's really easy to swap a dirty roll for a clean roll, I'll be looking to make the switch. It's a song and dance to get a new roll on the Clarisea without having to take it out or take it apart.
I've really liked the Nyos 160 skimmer I have on my 425 XL. It didn't need to "break in", I never have to play around with the water level, and it pulls a very dark skimmate. Everything comes apart so it's pretty easy to clean. It's a more expensive option but if you're exploring the Royal Exclusive than maybe it's worth taking a look.
Super excited to follow along on this!