OP is definitely dragging this out as much as possible. He's trying to get it fixed as cheaply as possible, while wasting everyone's time.
Since it's water damaged, DJI probably doesn't want to spend the time turning it on and trying to find out what actually went wrong. Obviously, they are smart not paying their tech to spend a couple of hours assessing the drone, cos OP is going to pay for their time. They might have cracked open the cover, saw the tell tale sign of water incursion and went, nope, just easier to replace the whole thing. Yes, they'll probably salvage whatever that's not damage, and recoup the cost of taking those bits out.
OP should just ask for his drone back from DJI. Send it to Thunderdrones. I think he'll give an assessment that most of us here trust as an honest assessment. The downside, if Thunderdrone do find that there's more to be fixed, and costs more than $300, the OP is going to kick a fuss about overcharging and wants his drone back, thus wasting Thunderdrone's time and $$$ of shipping.