First of all, I'm sorry to hear about your loss Avirunes. However, I'm relieved to hear that the amount stolen wasn't a fortune given that I've seen instances of people losing large amounts of Bitcoin due to being hacked. This year, in particular, has been especially tumultuous, and it's likely due to people being careless with their wallet security
I've read nearly every post here, and I was curious about how you got hacked, but it seems that you're not sure about it. Given that your wallet is connected to the internet, it could have been any things happened. You mentioned a script running in the background on your OS which could be the problem here as Malwarebytes may not have detected it
Ensure that your fresh operating system is clean and avoid installing any suspicious software that you are unfamiliar with(any cracked or random software) It would be best to use Linux, as it is generally more secure than Windows, but prioritize securing your BTC wallet. Personally, I use both Linux and Windows, and if I need to test out suspicious software, I rely on the
Windows sandbox feature, which acts exactly like a VM.