I think you're doing it wrong.Until level 5, Monks have a +10 to movement. That's what the spell Longstrider gives. It's okay, but not exactly game breaking. Monks get +15 ft. for level 5-9, which is a tad better but hardly game changing.
Monks pretty much have to be in melee to attack. To get in and out of combat and make use of their mobility, they need to burn 1 Ki point and spend a bonus action each time they want to do it. This brings down their DPR considerably.
Honestly if you want a high priority target taken down, you are way better off attacking at range. A Samurai, Battlemaster, or Gloom Stalker archer with Sharpshooter is going to do more damage more reliably than a Monk can ever hope for.
One thing is that you're making the implication that simply stepping outside of melee with enemies means they cannot attack you. That's silly. It's very easy for enemy creatures to move around, many even have extra movement capabilities.
Spending a bonus action and a ki on disengage is usually not a good idea. Spending it on dodge however is good. If you are attacked you are at least using up the enemy's attacks. And if they don't attack you well that's fine too.
Spending a bonus action on an unarmed strike or flurry of blows gives the Monk many chances to stun. A stunned enemy cannot attack.
Another flaw with your reasoning is that you're assuming a white room with vanilla creatures. There are all sorts of weird situations with weird creatures that pop up in D&D. Monks get a lot of choice on how they want to move around and interact with their environment. Ranged attackers also have the issue of either attacking with disadvantage in melee or eating an OA to get out. Ranged attacking is only perfect in a white room.