In addition to the other advice offered here I’d suggest looking up Harldan and Lonarra, two different channels of a speedrunner who’s made very in-depth guides to things like power leveling, making twink alts, etc.
Some great add-ons:
Narcissus - makes using gems LOADS easier. I’d consider this absolutely essential.
Remix Mail Helper Addon - Lets you individually retrieve non-XP mail, if you go the mail leveling route. Also calculates when you can open the mail to level.
Autolooter - You can blacklist XP items to loot everything but them from the boss.