OK, let's work this through:

If setting the rudervator surfaces up, evenly, a little allows the plane to fly straight then something about the attitude of the tail is wrong. I have this problem too, but I caused it. When I reset the tail I must have put it n slightly rotated to one side. So I have to have the right rudervator surface up sligtly when I set the trim to zero at the tail. I have two other planes, so I am not flying the Aerobird as much as I used to. This works for me. Frankly if the adjustment makes the bird fly straight, then that can work for you if you can't find and fix the problem.

As I understand it, both of your rudervators are up slightly and evenly to get it to fly right. So, surface adjust ment can compensate. This tells us for certain that we have an allignment problem which is putting the tail too low relative to the body, or the tail itself is mounted with the back of the tail down too far.

Here are the possible causes:

Boom Droop

When you epoxied the boom back in, it was not PERFECTLY straight. The tail portion must have been slightly down. This would cause the plane to nose down. You can fix this by remounting the boom, or raising the back of the tail slightly using the orange screws. Or, but setting the rudervators up slightly all the time, but the last is the least desirable.

Or, the body is slightly bent where the boom is attached so that, even if you put the boom into the body right, the body is bent slightly so that the boom is drooping relative to the motor thrust line.

Putting both surfaces up, or raising the back of the tail wing using the orange screws would compensate.

OR - Tail incidence is wrong.

Your tail is not adjusted properly. the front is too high or the back too low inducing a drag at the bottm rear, again causing the nose down ocndition. Putting the surfaces up would compensate. Following the adjustment procedures in the manual should help you make that adjustment. It takes a fair amount of time, patience and a calm day. There is no other way to do it. You have to fly it, adjust the orange screws and fly again. Anytime you take the tail off you have to do this. No way around it. I think this is where your problem lies. This would also be the easiest way to compensate for a slightly sagged boom that you don't want to reset.

You could have a center of gravity problem, but I doubt it would be this severe.

I am out of ideas. These are what I would focus upon. You say you did them and I am saying, based on your statements, this is where your problem lies. Do it again. Focus on the orange screws on the tail.