Honestly, id like to go a bit of a different direction, while keeping the spirit of the idea alive.
Instead of having the left most at 50 and changeable, keep it like it is.
Once you hit 500, make it similar to delves, where you can pick what level you fight at for the middle 1, so if you run through them all, get 500 no loss win, essentially 2,000 renown for that faction, you unlock the ability to change the level of the middle 1 you fight.
This way, you get the rewards increase that comes with it, cause you should not be punished with lower rewards for wanting to fight lower battles.
You can scale back the level to focus on pure faction delves and slowly increase the difficulty at your own pace.