Grounding conductor to sub panel from main panel & "main grounding conductor"

The whole grounding thing has me a little confused, here's what I think I understand as correct.......(I hope I'm using the correct name for all of these things.)
The grounding conductor from the main panel to grounding electrode must be continuous, no splices. The grounding conductor & neutral are only connected at main panel.....neutral & ground busses remain isolated at sub panels.
An underground metal water pipe (>20') is the preferred grounding electrode, if one is available. Grounding rods would be supplemental (?) & merely recommended in this case (?) as a backup in case the water line is replaced?
So ideally the grounding conductor would run from the main panel to each of the grounding electrodes (water pipe & ground rods) without being cut? Like doubled over at the rods on the way to the water service?
How about the sub panels?
One of my sub panels is located half way between the main panel & the front of the house where the 1" copper water pipe enters the house.
My old grounding system was a # 4 or # 6 connected to the galvanized piping at approximate midpoint of the house.....about 25' from water service entrance. But now all the galv is gone.
Prior to PEX / copper repipe, I drove a couple temporary ground rods at the main panel.
My plan is to finish the grounding system properly by running a # 4 from main panel to water service entrance,
The #4 run from main panel to water service entrance runs right passed one of my sub panels.
My idea was to split nut a grounding conductor from the sub panel onto the "main grounding conductor" along its run from main panel. Of course, not cutting the #4 "main grounding conductor" at this connection, just merely split nutting onto it.
This would bond the sub panel ground back to the main panel without having to run an separate conductor back to the main panel from the sub.
Does this make sense & is it correct (ok) to do what I am planning? Or do I have to do something else?
The whole grounding thing has got the be the most complicated & difficult part of electrical wiring........makes my head hurt.
cheers Bob
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It is confusing. Here's a distinction that helped my understanding considerably:
"Bonding" of metal parts that should never become energized is done with the EGC (Equipment Grounding Conductor), which is connected ultimately to the service neutral. This ensures that in the event such parts accidentally become energized, the OCPD (overcurrent protection device) will open. An earth connection can not perform this function for low voltage (120V-480V) systems, as the earth has too much impedance.
The entire bonding system is "earthed" to reduce the effect of accidental overvoltage, either from lightning or POCO (Power Company) error. This also eliminates any touch potential between the bonded metal parts and the earth.

The GEC (grounding electrode conductor) from the main panel to the first grounding electrode must be continuous, no splices. Additional electrodes can be connected via a bonding jumper to that electrode, or to the main panel.
Note that main panel here means the first OCPD after the POCO service. If you have an outside disconnect with a breaker, and an interior load center, then the outside disconnect is the "main panel", and the interior load center should be treated as a subpanel.

Right, every subpanel feeder needs to have a separate neutral and EGC in the feeder, and the neutral bar is isolated from the case of the subpanel.

An underground metal water pipe is a good grounding electrode, although there are issues with it. For use as an electrode, you must connect to it within 5' of its entrance into the building. The preferred grounding electrode nowadays is the CEE (Concrete Encased Electrode), i.e. a copper wire running to the rebar in your concrete foundation. For new construction, you are required to provide this electrode.

All grounding electrodes present are required to be connected to the grounding system. A metal water supply is required to have a supplemental ground rod, in case the water service is replaced with a nonmetallic service in the future. And a single ground rod requires a second ground rod unless you can prove that the ground impedance of the single rod is less than 25 ohms. In practice this measurement is much harder to do than driving a second ground rod, so if you are required to have a ground rod, you drive two.

It does not have to, whatever is convenient will work as long as all grounding electrodes are connected, and there is at least one unspliced GEC from the "main panel" to one grounding electrode.

Subpanels should basically be ignored for this discussion. You do have the option to connect a "supplemental" grounding electrode to a subpanel, but that is not defined.
Also, if you have a grounding electrode that is much closer to a subpanel than it is to the main panel, it would be tempting to have that EGC do double duty as the bonding jumper for that electrode. Whether this is permissible under the NEC is not clear to me, it is debatable.

As mentioned above, you need to connect within 5' of the water service entrance.

#4 Cu is a good choice as long as your service entrance conductors are 3/0 Cu or smaller [or 4/0 Al or smaller]. You can run the #4 Cu as a bare wire as long as it is not exposed to "severe physical damage".

The subpanel should already have a 4 wire feeder including an EGC, so it should already be bonded to the main panel via the EGC. As I mentioned it is a grey area whether you can use the subpanel feeder EGC as a bonding jumper for the grounding electrode. I suggest avoiding that issue and just running a separate bonding jumper from the water service entrance to the main panel.
Hope this helps.
Cheers, Wayne
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