It's not just about the energy, though, even if I think it's a bigger problem than you do. If you get up to a decent speed where time dilation helps, then the interstellar medium becomes a serious problem. I think there were some other weird problems, too.
I'm not saying interstellar travel is impossible, I'm just saying an early technological civilization in our solar system is more possible.
I'm not sure about that. Earlier life? Almost certainly I think, I tend towards panspermia on space rocks as being likely at least within the solar system.
I think the likelihood that there was some technological civilization that evolved here before ours and we haven't found any trace to be extremely low. Lower than "there are people with warp drives who visited our solar system at some point".
I see your numbers, but I still don't understand the reasoning. We know technological civilizations can exist, the only question is the timeline. We don't know that interstellar travel is feasible under any timeline at all. How can you rate the second one more likely?
(And as far as detecting relict machines: our astronomy is really not that thorough. They wouldn't even have to be hiding. Have we conclusively analyzed every little point of light in the asteroid belt? No.)
I'm not saying interstellar travel is impossible, I'm just saying an early technological civilization in our solar system is more possible.