Best Of
Re: Zone Valves BANG on opening only
Remove the check valve on the circulator with the zone valves.

Re: Am I being dense?
or replace green valves with these reversible ported shutoff valves….
https://www.supplyhouse.com/Webstone-80613-3-4-Press-Full-Port-Ball-Valve-w-Drain-Reversible-Handle
Red ones doesn’t hurt as @psb75 pointed out but removing may be cleaner.

Re: What I think
I'm not a heating pro, just a homeowner/engineer who likes to build and fix things. I was born in 1964, and both my parents went through the Great Depression as children. We weren't poor growing up, but their "waste not, want not" ethos from living through times of scarcity got ingrained in me.
So I hate seeing good equipment thrown in the trash because something minor is wrong with it. I've lost count of how many lawn mowers, weed whackers, snow blowers, etc I've taken from curbside trash piles and fixed, with only minor repairs needed to make them run.
@Teemok said "Many companies are little more than wealth extraction rackets in service costumes." Last year I tried to get pricing for converting our oil burners to gas burners. One such "service" company sent a plumber out at my request to price out installing about 30 feet of gas line from our existing gas meter to the boilers. Since we can't quote prices here, I'll just say that his price quote was about half of what you'd pay for a new car. For 30 feet of gas line that would take him maybe a day to run.
I didn't want to insult him, as he was probably only doing what his boss told him to do. So I thanked him and told him I'd pass his quote on to the other condo owners for review. But these guys obviously had a business model that was basically "quote a ridiculously high price and hope that someone is dumb or desperate enough to bite." And they must be getting enough customers to pay those ridiculous prices, or they wouldn't be doing it.
So I feel for customers who aren't knowledgeable enough to sort out the shysters and crooks from the guys who will do a good job at a reasonable price. It's a war zone out there.

Re: What I think
Shady companies wouldn't be able to make it if consumers didn't buy what they were selling.
Thinking just isn't very popular right now.

Re: What I think
I've been a repairing /diagnostic tech for decades regardless of who I worked for. The modern company modus operendi puts a proper tech continuously in the conflict between good ethics and the maximum profit seeking of the company. Technical and diagnostic skills are valued well bellow a predatory company loyalty. Many companies are little more than wealth extraction rackets in service costumes. The managers are Fagin like and the top techs Art-full Dodgers in one form or another. I found myself forced into a 50% warranty role, cleaning up the repeated messes created by our own operations with no means of correcting the root causes of the problems.
I used to be more sure that I could run profitable company on a foundation of excellent diagnostic skills and honesty. Customers are more and more unfamiliar with the repair ethic and sometime react suspiciously to it. What's this guys angle, they ask themselves. They are somewhat convinced the flashy spoils of exploitation are real signs of excellence. Like the jewelry of the con man. The sad truth is, replacement, if done well, is often a better option for the customer than dealing with the poor results and multiple invoices of crap techs. Lowered customer expectations and the normalization of ill feeds the cycle to lower places. The more people move around the less value a good reputation has. I wounder how long before the venture capital firms just own their own techs to service their ownership of the bulk of the housing market.

What I think
Seems like in a lot of posts recently I notice a common theme or maybe it's just me.
Customer calls for service and the "Tech" shows up. I use that term loosely.
He looks at the boiler/furnace/air conditioner/heat pump/water heater and says:
"You need a new one"
and then leaves the job
It's no wonder that homeowners hate contractors. Can't blame them
Re: CH System Overhaul with Cast Iron to PEX Conversion
I believe Taco makes a ODR mixing valve! You set the ODR from the boiler to operate the high temperature zone and you set the Taco mixing valve ODR for the low temperature zone. Basically you’re resetting the low temperature zone off of the already reset high temperature zone.
Re: CH System Overhaul with Cast Iron to PEX Conversion
A mod con is probably a better boiler for that system, you can run odr on it set for the ci radiators then mix it down for the other loads but the mass of the system is likely something you'd need some protection from low return water temps on a ci boiler. Might want to use a control to do the low temp zones that can do odr too.
Do you really need 170 to the CI ever? It is possible you might but is likely you don't.

Re: Noise complaint: can I lower the pressure on my system?
Just from your boiler size I can be pretty sure that vent would be well undersized. To properly know what you need you would need to get an estimate of the total length of your main and its nominal pipe size(s).