I guess the perfect build to match all your needs is Monk>Zealot>Plague Doctor, but only after the classes receive their rework (which will come in 1-2 months from now).
First of all, Monk allows you to hit enemies over a certain range with quick attack flurries and has the ability to use Energy Blast (which is basically equivalent to Dragonballs Kamehameha) that knocks enemies back&down. Energy Blast can also be spammed when certain conditions are met via an art, further improving range, AoE and damage of the skill.
If you don’t like to manage too many buffs, this class is for you, because it only has 3 of them, one lasts forever(till you change the map), one lasts 30 minutes and one lasts 30 seconds after the change when used at the right time.
The Nirvana buff required to spam Energy Blast lasts 15 seconds if you like it, but you don’t have to use the skill/art at all.
Zealot is a nice damage dealer because all the damage it deals is buff&debuff based, so you can move freely while you deal damage. It has a nice damage buff that lasts for 30 minutes after the changes, and some optional boosts to crit damage/crit rate if needed.
Plague Doctor is mainly a support class that is put on this build for Modafinil, which will heavily increase your movement speed (high mobility), and Beak Mask to resist status ailments.
You also have access to damaging skills that are debuff-based, meaning you’re free to run around while they continuously deal damage.
Since Modafinil works for the whole party, your brother is free to take a different support class aside of Priest (probably Diev & Oracle) while you still have access to high mobility.
Note that this build is by no means one that reaches top tier, but you might have some fun with it.