The most important check and balance is the people themselves, but they’ve been routed. Keep in mind that when most people say, ‘checks and balances’ they’re only thinking of the federal level and the common Executive/Legislative/Judicial distinction.
The most critical check and balance is between elites - federal v. state. The 17th Amendment and arguably the 14th and 16th, but critically the 17th unbalanced all of that and placed the states in a subservient position.
That the people in general at the local, state and federal level tolerate an executive that legislates and adjudicates tells you how far our education system has fallen.
State governors have allowed the federal executive and Congress to usurp powers that are theirs under the tenth amendment. To reverse this they must organize and fight to restore those powers.