Hachiman Conquest (Y10) (ADC)

You can absolutely level Eagle Eye if you find it's more useful to you. I specifically noted that leveling Iaijutsu is for better safety, and I will point out again that it gives you additional range. If you get both hits of it you'll deal a base 80 damage, which is 40 more than 4 hits of Eagle Eye's bonus damage (but not including the basic attack damage you'd also connect with if you do hit them). I will just point out that most people do in fact grab Iai 2nd.

Same with leveling Heavenly Banner. Most will max it when possible, or even at level 9 before putting a 2nd point into the ult. Zapman did that here, along with grabbing Iai 2nd. Now, I'm not saying you need to do what the pros do, but there are also reasons they typically do these things and they make sense. Again, you're welcome to do as you prefer, and I'm just providing comments with my recommendations.

Griffon is one of the items I'm undecided about at this time. Even when it was popular and had stronger stats, I would almost always only pick it up on 3 gods... Hachiman, Ishtar, and Freya (sometimes). I can agree I like it especially for Hachi's 1, but I do feel its stats can be generally found on other items, while also providing other important stats (like more pen, for example). I'm fine with building it on him, but what I was arguing was not purchasing it, but where you purchase it. At the point of the build you have it in, right after Rage and before The Executioner, I would get Exe first. Objective shred will be important at that stage, and if you're taking GF for example and you have another phys teammate there (e.g. Assassin Jungler), they'll get more damage out also. Or you can grab Dom there and deal better structure damage due to the % pen.

100% if you grabbed Rage, you should just finish with Deathbringer rather than a placeholder like HS.