Couple of points:
Do NOT saw cut crack control joints into the slab! You don't really know where the tubing is.
Probably the best rebar spacing for doing radiant is 1' on center. 9" OC heats better but is harder to pour. Wider spacing than 1' will give poor results. Tie the PEX directly to the upper bars with rebar ties.
Try to make all loops approximately the same length and no longer than 300 linear feet. Shorter with more loops is better. More tubing in the bathroom is better too. Make kind of a figure 8 pattern and end up with twice and much tubing per sq ft., or 6" OC. A double pass at 6" or with two tubes next to each other near exterior doors will allow the heat to feel more even and greet you when you enter.
Hold the PEX back from the perimeter about a foot or more and insulate the perimeter of the slab.
If you have dry ground under the slab, I recommend not using insulation there. If someone says you'll have a lot of heat loss underneath, ask them where it will go and what the mechanism is.
The cast iron boiler must be allowed to run above it's condensing temperature. Forget the motorized electric valves for mixing the boiler water and just use a simple T arrangement with a couple of ball valves, where some of the water bypasses the boiler as it returns directly to the pump and back to the floor. Bypassing the boiler allows it to come up to temp and it should run at about 130-140 degrees as controlled by it's own high limit, just high enough to keep it from condensing. Of course the slab delivery water can be much cooler than that. Your boiler BTU rating (output in this case), should be about 25 BTU/sq ft or more. If it is a single thermostat zone, 50 BTU/sq ft would be plenty. Bigger yet starts to lead to a lot of cycling and possible room temperature overshoot. The average floor temp will feel nice at about 70 degrees for living there. You'll never need it to be more than 85 degrees even in the bathroom. A 60 degree floor is nice for working as a shop. The delivery water from the boiler must be warmer than 90 degrees but need not be warmer than about 110 or so. All temperatures are variable over time from the boiler and variable across the floor as it comes up. Proper heating is done with high flow rates. Don't try to get rid of all the heat in the delivery piping as it returns from the slab, but try to make the supply and return temps as close as practical to each other and all loops equal. You should see about a 10-20 degree temp drop as the heating settles in and progresses. The closer the supply and return temps, the better the heating, meaning faster and more even.
Control the whole system with a normal setback wall thermostat so you can have a timed run and a temperature run each day if desired with a minimum temp set point for the middle of the night. A larger boiler will heat the system faster, but you will get overshoot if you don't control it carefully. Best to not run it with a simple non programmable thermostat. If you want freeze protection from the boiler, you can install a slab sensor and a controller that fires the boiler at a pre-determined low limit. Some boilers also have a freeze control designed in that you could also use. You might leave it on continuous circulation and have the boiler fire up at 38 degrees, for instance. But during a long power failure the antifreeze is your friend.
If you are worried about freezing the slab while it is shut down for long periods in the winter, Propylene glycol can be used. It's compatible with PEX, iron boilers and pumps. Mix a minimum of 30% and no more than 50%. The first year I filled mine, I got nervous when the slab got down to about 39 degrees. I had a thermometer laying on the floor and watched it. Then I fired the boiler and brought it up. Now, if we have clouds and snow, like today, and the doors are open most of the day, the solar stays off and I lose about 2 degrees or so. A full sunny May day at this elevation and warm, brings it up about 2-3 degrees.
I live at 5,000 ft and it gets fairly cold, but I am running straight water in mine, with a little bit of corrosion inhibitor. I have an oil fired, cast iron boiler. My slab is 8" thick and about 2,800 sq ft. I used 3/4" PEX throughout (7/8"OD) on 12" centers. I put a grid of #3 bar on 12" centers in both directions. No insulation under the slab, but 2 ft of vertical insulation, (2" Blueboard) around the perimeter, inside the stem wall. Rebar came out of the stem wall vertically and bent over to tie to the slab bars. I backfilled with gravel up to the top of the stem walls and top edge of the insulation. Then capped the whole thing with an 8" slab. The radiant is closed loop with no connection to the domestic water, no air vents, no air scoops, no conventional expansion tanks and no mixing valves. None needed. I simply put a holding tank on the second floor loft area and poured in the water by hand. It is about an 8 gallon tank about 1/2 full with a small line running down to the boiler. All air accumulates up there by gravity and half the volume acts as the expansion tank. It is a completely closed system. I also put a heat exchanger in the circuit and heat that with solar panels. This last year we used no oil at all. So, it heats very well with solar only, but uses oil as a backup if needed. I am currently installing (5) 120 gallon storage tanks to store solar for night use and domestic hot water, as the system has been running for two years with the collectors going straight to the heat exchanger (when needed) and the floor circulator running continuously.
The thing that makes radiant so efficient is its low temperature operation and no pumping of air against cold surfaces or losses through ducts and other air leaks. It is also the most compatible with solar because it runs at the coolest temperatures. But the best part is the comfort. The conversation always rolls around to recovery time and how to compensate for slow heating. Radiant doesn't work the same or feel the same as forced air, so it's hard to measure radiant against air in recovery times. In my experience that is never a problem with correct programming. Also, the room temp can be quite low, but very comfortable if the floor is warm. So, it's not really the room temp that we are striving for. It's comfort. If needed, a wifi thermostat can be installed to allow you to turn on the system before you arrive.
