Not a Bitcoin Core user and never has this experience.
To my knowledge if someone have compromised your wallet, didn't they can just spent all the coins you have now? so I think your computer is the problem. Why you not try to use other device and input the seed phrase, I think as long as it never connected with your computer, the device should be safe.
Exactly that was what I was also thinking. If it was hacked or compromised, the total amount would have been swept up by now. because no body will see such amount without withdrawing it. I guess there where extra charges from the transaction he made just as
Faisal2202 has explained.