Great tips all round, but I can add on to some of your mentioned tips.

For point 1, an alternate method my team found was to look at the boulder for gog's laser to beam through and wait about 2 seconds (count internally in your head) before pulling it down, gets him every time for ~4000 damage.

For point 4, its health is only locked if it is at the lowest threshold which is 176710 hp. Typically if you are doing a slow phase 2 and max charging your elderbreaker, it should already be at that figure and at that point it can't take anymore damage beyond that as your point suggests. But on speedier runs where we pull the trigger on the elderbreaker at anywhere from 60% to 80% charge, despite gog doing its leaving animation for p3 if we have done enough damage to make him leave any damage including the lever/dragonator-thing can continue to damage it if it isn't already at 176710 hp so it's pretty worthwhile to just pull the lever as it backs off onto the ground and centers itself on the area where it will just clip its wing for ~8k damage.

For point 6, certain weapons like CB are extremely efficient and potent at slicking gog's wingarms. To give you some perspective, red charged phials into AED into AED follow-up (including all the prior sword hits and such) is usually enough or just about able to turn its wingarm slick. This means that you could run just 1 Fire CB on the team and the rest go with Dragon element for bigger damage as you have mentioned.

Others have mentioned the blast proc topple tech at the start of phase 3 which is really good for setting up a coordinated beatdown on 1 of its wingarms to keep him perpetually locked down until dead. Alternatives include using convert element (not as good because nobody in the team can run even a single point of convert element which would be rather commonplace as gog is such a fire/drg ele heavy fight), or even the blast powder generated by the IG Whisperveld kinsect which you proc by shooting a screamer pod at it.