That only works if the attacker doesn't know the guard node is you.
If they do all you've done is made the middle mode the guard.
If they do all you've done is made the middle mode the guard.
That's not how Tor nodes work. Once you setup a guard node (and it got enough reputation) you won't be the only person using the guard node. Also de-anonymization attacks require you to know the traffic coming to the guard node (and if you run a trustworthy one yourself and you're not dealing with a global passive adversary then there's no way the attacker will be able to see the incoming traffic to the guard node).