Calcium reactor, at it's core, is just a container that recirculates water. All of them have more in common than they do not. Everybody has their favorites.
I personally like the co2 to get trapped in the top. This lets me know that my tune is good or bad - if you have a good, efficient tune, then there will be no undissolved co2 going into your tank and the reactor will produce as efficiently as possible and nothing will build up. Because of this, I like reverse flow reactors to pull from the top. If I have a reactor that does not trap, I add a short tube so that it does.
I do not need a second chamber because of the above... I tune them correctly so that there is no need for one. Second chamber only has benefit to remove excess co2. If there is no excess co2 coming out of the reactor, then it does nothing.
If you have a well tuned CaRx, then your tank pH should barely more... if you have a lot of extra co2, then it can lower quite a bit.
Precision needle valve is very important. CarbonDoser is one version that has an electronic knob. You can also buy them pre made. You can also get a regular-old regulator and spend $50 on a precision needle valve. I have found all regulators to be about the same for reliability, the only difference the ability to fine tune the needle valve.
Be sure and use a high quality check valve or else saltwater can creep back up into your expensive regulator and cause it to start to corrode. Solenoid can help this in cases of electrical outages, but they stay open when bottles run empty and stuff. You still need a check valve with a solenoid.
I have no idea what my pH is in my CaRx and have not known for more than a decade. I tune them using the effluent dKh only and watch for gas buildup in the top. I do not use a pH controller. I do not recommend using a pH controller.
I am currently running a pair of Korallins (probably my favorite since I do not need to modify them), a AquaMaxx (won it a show auction and like it, but I needed to modify it to trap gas) and a really old Knop. The knop does not need fed, but I feed the others with QuietOne 1200 or MaxiJet pumps - I have used T off of the main line before too. I have a pair of Victors a Tunze and a M3 regulators on the tanks - all of these are very precise and hold their tune. I could probably use any RELIABLE kind that is reverse flow and be happy... these are not complicated creatures.
Whether you want to dose 2/3 part, use kalk or have a CaRx is up to you. Eventually, you might get to a point where you cannot evaporate enough water to keep up with kalk, even with acid in it... this can happen early in a SPS tank's life and especially if you have a lot of coralline. My tank would need a slurry added to it and the pH might be near 9 to get enough in there to meet demand. Most people do not get here. I like a CaRx because once I figured out how they worked 20 years ago, I can pretty much just look at it and tell if it is OK or what it needs. Other than changing media every few months and a bottle less often, they need nothing and just run and run. I also like that if I melt natural media, I am introducing a whole bunch of different traces that the coral uptook when they grew... more than just carbonate, calcium and magnesium, but about a dozen others (reborn and ARM have their assays out there to view).