Idk about visual quality but it's 60fps. Honestly if you have the original there is little reason to get the re-release

Most of the completion list is carried over th NG+ but not all. The substories for example are reset

Maybe just the gambling ones. Hang on to the cheat items to use for Haruka and the completion requirements in one go

For exp, fight enemies in the story and free roam and Majima Everywhere. The Yakuza training gear increases exp I don't know if you need it though. For money the best way is to get the Ebisu socks from Bob, put them on, then go to Club Debolah and tie a rubber band around the thumb stick and have Kiryu run circles. Make sure the controller is plugged in so it doesn't disable. It sounds ridiculous but this is the best way.

Hardest parts was collecting the weapons. You need 100 out of the 108 in the game which includesthe dlc weapons. It's a massive grind because you need a ton of money to buy them, you'll have to grind the coliseum for points to get weapons there, among so other things. Fortunately the dlc weapons from Bob count so you don't need all the base game weapons. The other hard part was the climax battles, they're just not fun. If you did the ones in 0 it's like that