What checks remain to counter this power? Impeachment? Constitutional amendment? As I understand it, if the president chooses not to enforce a law, then the only real recourse Congress has is a massive escalation that requires an extremely high level of cooperation. I'm not sure it was ever intended for the executive branch to simply ignore the other two branches and unilaterally decide how to run things. Personally I think willfully refusing to enforce the law of the land should be an impeachable offense but I guess that's not how it works.

The judicial and executive branches are checks on the legislative branch. The entire point of a check is that it can't be overridden. If the judicial branch determines that a law is unconstitutional or the executive branch determines that it should not be enforced, than that's it; it's dead.

The legislative branch can try again with another law, but if it doesn't change whatever made the law unconstitutional or detrimental to enforce, than the relevant branch will keep it dead.

The only condition in which the judicial branch regularly forces the executive branch to enforce laws is when the executive branch tries to legislate through selective enforcement; then the judicial branch will give an all-or-nothing ultimatum, but even then not enforcing is an option, just not selective enforcement.