Mixing Valve Question

I bought a 1778 house about 13 years ago, and converted the first floor heat from baseboard to radiant heat with a boiler dedicated to that system, installing Brazilian cherry floors with the tubing in the subflooring. I designed and installed the system myself after reading “Pumping Away” and “Primary and Secondary Pumping Made Easy” (Thanks, Dan!). It has five zones and has been a great system for all these years. Unfortunately, the water jacket in the boiler has cracked and I am faced with replacing the boiler. As long as I am taking the system down to do this, there is one problem that I need some advice on. The local water is full of minerals, and deposition is a real issue, although I have had to replace only two of the six circulator pumps on the system after eleven or twelve years or so. I believe that the mixing valve suffers the worse from the corrosion and mineral depostion. I needed to replace the first mixing valve after about two years (I don’t recollect why), and the second mixing valve froze at whatever setting it was on a couple of years after that. I have inline water temperature gauges in the secondary circuit before the mixing valve, and on both the outflow and the return to the mixing valve. So here is my question: With a set up like that, do I really need a commercial mixing valve, which seems to be the weak spot in the whole system? Is there a reason why I cannot simply install two or three high-quality ball valves and manually do the mixing by opening or closing the valves incrementally, using the temperature gauges to guide me in this and adjusting the flow with the same, if not better, precision than a commercial mixing valve? Also, if there has been no trouble with it, is it necessary, or even good form, to replace the expansion tank? I appreciate your input on this. Thanks!