Adjusting the drain & repair rate multiple times will not change the outcome. Some other part of the mechanic needs to change.
If you make it easier for attackers to drain, you will have defenders complaining that a Site are getting drained too quickly without being able to react or defend against Tank class, which is a fair point. Sites are being taken without a fight.
This will lead to the Site holders to not upgrade their Sites to yields that are drained too fast.
Now you will have attackers complaining that they are unable to take Sites because there are too much defenders and Sites are taking too long to be drained.
In the end, constantly changing the drain and repair rate will lead to an endless loop of complaints from both sides, with goals unmet.
I believe that you should find a midpoint. That midpoint was reached when the "Excellent" yield site were being drained in not more than 4 minute. (I forgot the drain rate and repair rate that it was)
The next adjustment in the mechanic needs to be in the mobility and freedom of the attackers. If an attacker dies, what are his options?
Let's take Eu1 as an example since the discussion has been mainly about that server.
In Eu1, the majority of sites are still held by the defenders, leaving attackers only selected revive gates, where they are immediately killed by the camping defenders.
But what if you give the attackers options to revive in any gate, while the maximum drain time of an excellent building is 4 minutes?
Who will complain?
Defenders cannot complain, 4 minutes is enough time to react.
Attackers cannot complains, they have more revive options and they can spawn anywhere they wish to.
I think you should give attackers the ability to teleport in any site while having duelists.
Attackers are severely still limited in terms of movement.
Once they get killed, they have nowhere to respawn except for some selected site that DV holds.