> That only works if the attacker doesn't know the guard node is you.

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).