Background: 1950s single story ranch style house in California, late 1960s split bus electrical panel (100A service?), outlets in house are seemingly all 2-prong ungrounded. Electrical panel is on outside of unfinished garage. Garage upper part connects to attic space over adjacent kitchen/laundry room area, and that seems to be where cable has been run. Am currently in the process of installing 3x 8' copper grounding rods tied with 4 gauge bare copper as grounding electrode system to electrical panel to functionally replace old single galvanized pipe/10 gauge(?) copper wire.
Goal: Incrementally add "real" EGC to outlets (convert to 3-prong), starting with kitchen counter outlets, laundry room outlets, and refrigerator outlet.
Plan: I can't do a full-blown replacement of cable from the panel to each receptacle at this point. I do want to make the kitchen/laundry rooms safer, so I plan to run a single EGC grounding "bus" (6 gauge bare armored grounding wire) from the panel up and over, through the attic space along joists. This would connect to standard square steel junction boxes above each of the 4 outlets (2 kitchen counter, 1 refrigerator wall, 1 laundry wall). I would then run 10 gauge stranded green THHN wire from each junction box down through the wall cavity to the receptacle box and new 3-prong receptacle (existing hot/neutral + new ground wire). Kitchen counter receptacles would be GFCI, while laundry room (washing machine) and refrigerator outlets would be standard 3-prong.
Qs: Is this plan compliant with current code in the use of a common bare armored grounding wire "bus" to ground multiple circuits (2 I think)? Am I OK with 10 gauge THHN running from the grounding wire bus inside walls to the outlets (no conduit)? Does my grounding wire "bus" wire need to be unbroken, or can I make standard wire nut connections with the THHN inside the junction boxes? Are there safety issues I might be missing here? Also, should I somehow mark the outside of the armored cable to indicate it is an EGC - with green tape perhaps?
Bonus question: Laundry outlet is within a foot of the copper overpressure drain pipe from the water heater. Hot and cold water lines and black steel gas line are bonded together, but not connected to electrical ground system. Should I extend the gas/hot/cold bond to the copper overpressure pipe, and connect that with a 10 gauge THHN wire to the grounding "bus" to link the grounding systems?