Isn't it possible to just start a new game to level up another character?
I too don't like games that drag on for too long. Unless it's something like The Division, where your campaign character sort of carries on into the online modes, which is where you sink most of your time into...
You get a bunch of characters that are unlocked during the story and find other random ones with the loot boxes you open. So you unlock several characters by just playing the game but these are generally weaker than the random ones you find. You'll get more characters that you can level up so you have to choose a core team of 4 and focus on them first.
There are also different tiers of gear you can use with different stats which are RNG drops. Gear gives you like half or more of your power so you'd lose out on all that if starting a new account. Also better stuff drops if you're a higher level. I'm at adventure rank 48 at the moment. The really good stuff mainly starts dropping at adventure rank 45. So playing at a higher level is much more efficient at gearing up new characters.
The Co-Op is quite nice. It's the first non-board game I'm playing online with my siblings. Partying up twice a week to kill some weekly bosses together is fun.