Can somebody help me track this down? Where did my coins go and how can I recover them?
If the transaction is already confirmed there's no way to recover them back.
What is the current receiving address from your Marathonbet account? Can you copy that address and paste it here. So that we can check if the address receives BTC last January 5th.
Do you have any AV installed on your PC?
Have you recently verify the Electrum signature before you install?
For now, there 's no way to recover a BTC if the transaction has more than 0 confirmations.
Much better scan the whole PC with Malwarebytes and updated AV to check if your PC is infected or not.
Also, check your browser if there is any installed plugin/extension that might cause of "auto-change address" when pasting.
According to your post on sportsbookreview it seems that you sent the BTC on Bitcoin cash or BCH?
If that's the case only support can help you out about this issue because they are the only ones who have access to this wallet. If they can retrieve or get the private key of that Bitcoin cash address they can retrieve your BTC.
Where did you get 19rW1dFKYog8WsthwfaghsWAWM7wJioze4 from? Who told you to send your funds to this address?
This is the weird part that you receive the change BTC to random address it should be sent to the same bech32 address as changed address.