Support at a few other places didn't even know what I was talking about, and didn't see the difference between uploading cost basis and simply reimporting old trades...
Well, it was clear from the 5th minute they don't have "Deribit support", and I had many email exchanges with them spelling out in detail how their engine misunderstands my transactions (cost basis etc.). I gave them every opportunity to improve, and instead they eventually just ghosted me.
I understand it may be unrealistic to expect they will build this capability in three weeks. It's a shame they weren't able to level with me, and talk about the complexity of this. I ended up writing a taxable gains calculator for Deribit myself and would be happy to share it with them.