Personally I feel like he got easier in each subsequent game as Sega improved the battle system. As you got more control and moves the fight against Amon got easier. Dead Souls Amon fight is definitely the easiest in the entire series though.
That being said, it’s worth fighting Amon in all games as it helps round out his character.
The best thing Yakuza does (imo) is the story and recurring characters. If you’re completing every side mission from beginning to end you’re gonna start noticing a ton of people that come back in each Yakuza title. Even just little 1 shot guys that only appear in a single sub story in Yakuza 1 come back in Yakuza 3.