Advise on how to remove defective valve

I'm pretty handy, but I don't do much plumbing work. I have a sprinkler valve that failed. I've been draining my system for 7 years without issue, and have no idea what I did wrong. However, as you can see, there is a crack in the valve housing (see "CRACK" in the picture). It has a mild drip with the valve open, when I close that valve it sprayed quite a bit out the crack (there is a shutoff in the house so I just closed that for now).

I've put numbers on the different fittings (each side). I did attempt to loosen things up, but found that the pipes going into the house or ground flexed a bit so I'm seeking advice before doing anything.

So clearly I want to turn nut #4 while holding nut #5 it seems. I think I would turn 4 counter clockwise, i.e. put a wrench on the left side and pull it towards me while putting a wrench on #5 holding it firm.

Then I wondered if the fittings above #6 and to the left of #3 are designed to turn or are they tight? Because when I turn #4, that's going to turn the entire valve, the shutoff handle is going to rotate towards the house. So I need to know the interplay between these fittings.

Or, do I need to break the union of #1 and #2 so that #4 and #5 can rotate freely?

I thank you in advance for your help.

NO unions on that to take apart. you will have to
#1 cut the copper pipe.
#2 Rotate the Back Flow Valve to bring upper ball valve away from wall.
hold 5 and turn BFV
#3 remove upper ball valve at your 3
#4 remove cracked valve. hold 5 with wrench and turn 4.

if BVF can not rotate 360 degrees because it's to close to wall you going to have to cut it out. cut the cracked valve above your 4 in the groove.
will require soldering to put back together the #1 cut

Not the answer I wanted But I do appreciate you taking the time to explain it. A few more questions, please.

So I'm removing upper ball valve so that then I can turn the backflow valve and loosen it from 5, correct?

As far as cutting the pipe, I understand what you're saying. But if I turned 4 and 6 at the same time, while holding 5, wouldn't that loosen both? Or is the reality that it's too much torque on the whole setup?

after you break the plastic pipe while trying to turn 4 and 6 at the same time. then just cut the plastic pipe below where ever it is you break it

as Mr David suggested

There is no plastic pipe, everything is copper. Trying to understand, that's all. I have what I would need to solder, it's just something I've avoided doing so far in my DIY endeavors. My father in-law, now 94, used to do those types of things for me. Given the simplicity of the connection though, I'm sure I could get it done if that's my only option.

i suggest, to add a union where you cut, 2 reasons, easier to solider, and next time, easier to take off

once you remove the upper ball valve by turning the back flow and cracked BV as a unit at 5, If the back flow and lower ball valve can spin 360 degrees with out hitting the wall, then just unscrew it from 5 as 1 unit. Once the unit is off remove the cracked ball valve from the backflow valve. MY upper #2 turn at top of backflow valve was meant to mean turn both as a unit including the upper ball valve so you can remove the upper BV

Thank you Mr. David, I understand what you're saying and appreciate the instructions.

Thank you Frodo for your suggestion to install a union, makes sense for future easy of removal.

After I repair the garage door spring that just snapped yesterday afternoon (rain and pouring), this is next on my list.