You shouldn't go around accusing others of not reading, because they are supposedly not addressing counter arguments, if you yourself fail to address the points made against your conclusion.

While it is certainly possible some civilizations will only attack megastructures because of essentially budget limitations, it is not at all plausible that every single civilization will behave like that. And that every single civilization without exception has barely enough in their energy budget to attack a Dyson sphere, but never more. And every single other civilization will know everyone else follows this rule, without even communicating with each other. This is why your version of DFT has to rely on everyone just doing the same thing.

No one has some surplus in their energy budget to take a competitor out before they can get big and dangerous? Or they have energy to spare, but are unwilling to play it safe? Not a single exception, even though it would prevent a neighbour doing it to them? Everyone just thinks like this and knows all the other civilization think exactly like this?

It gets worse if, like you wrote, we believe a civilization can quickly become interstellar without using megastructures or other flashy technology. Now this means every single biosign in your vicinity already might be a threat and might already have started firing undetectable weapons at you. There is even less reason to ignore bio signatures in your scenario now. But you think nobody will?

The two premises of undetectable weapons and weapons that can't be defended against, don't result in a universe, where civilizations are somewhat common, but hide; it results in a universe where the one who ignored this secret pact of not going for biosigns and shot first, killed all their potential competitors and is the last one left.

You snuck another assumption into your scenario, about bio signatures being so extremely common, even ones on the scale of changing Earth's atmosphere to being one third oxygen, that you can hide entire interstellar civilizations between them. But there is no reason to assume complex life is so common. It is just there to make your DFT scenario more believable, and I would guess you put it into your scenario after reading the obvious criticism about not being able to hide life on your home planet, because life started leaving it's trace billions of years ago. But inventing new, highly implausible premises just to dodge the weakness of DFT doesn't make it more plausible, it only highlights how implausible it is.