How long should I stay at one rank before calling myself hardstuck?

A few hundred games at the same rank with no upward drift is a real plateau. A bad week is not. Rocket League MMR swings enough that anything shorter is mostly noise.

Should I grind freeplay or play more ranked?

If you can already hit the shot in freeplay and still miss it in a game, more freeplay is not the fix. The gap is decision making under pressure, and that only shows up in real matches.

Does watching pro players help?

Less than people expect at Gold through Diamond. Pros solve problems you do not have yet. Watching your own games is slower and far more useful, because the mistakes are yours.

Why you're hardstuck, and why it isn't your mechanics

The frustrating part about being hardstuck is that it does not feel like you are missing anything. You know the mechanics. You can hit the shots in freeplay. You watch the tutorials. And you still lose games you feel like you should win.

That feeling is usually accurate, and it is the reason the plateau lasts. The thing capping your rank is generally not a skill you lack. It is a habit you already have, running automatically, that you cannot see while you are playing.

Mechanics are not usually the ceiling

Below Champion, most matches are not decided by who can air dribble. They are decided by who gave the ball away, who was out of position when it mattered, and who ran out of boost at the wrong moment. Those are cheap mistakes in the sense that fixing them costs no mechanical skill at all, and expensive in the sense that they decide games.

This is why the "just grind freeplay" advice stalls out. Freeplay trains the part of your game that is already good enough to rank up. It does nothing for the part that is actually losing you games, because that part only exists when there is a ball you might lose and a teammate you might collide with.

The habits that actually cap a rank

The specific leaks vary, but a small number of them account for most of the damage in the Gold through Diamond range.

Ball chasing dressed up as rotation. You believe you are rotating back. On rewatch you are drifting toward the ball on most of your retreats, arriving in the wrong place slightly early, and getting scored on behind you. This is the single most common one, and almost nobody believes it about themselves until they watch it happen.
Challenging fights you have already lost. Going for a 50 when your teammate is already there, or when you are clearly second to the ball. Each one costs a possession and often a goal.
Playing on no boost. Not a mechanical problem, a resource problem. On twelve boost you lose every duel you enter, and it feels like the other player is simply better.
Double commits. You and your teammate both go. Whoever wins the ball, your half of the field is now empty.
Kickoff habits. Same approach every time, same result every time, and it is a coin flip you keep losing.

Notice that none of these are exciting. That is the point. The stuff keeping you out of the next rank is almost always boring, which is exactly why it survives so long. Nobody makes a highlight reel about grabbing a corner pad.

Why you cannot see your own leaks

While you are playing you have no spare attention. You are tracking the ball, your car, your boost, two or four other cars, and the clock. Every one of your habits runs in the background without asking permission, which is the definition of a habit and the reason it is invisible.

Watching your own replay is different, because you have attention to spare. The catch is that most people watch a replay the same way they play it: eyes on the ball, rage at the goals against. That tells you nothing you did not already know.

How to actually find yours

The trick is to stop looking for mistakes and start looking for patterns. One bad challenge is variance. The same bad challenge six times in one match is your ceiling.

Pick a loss, not a win. Preferably one that felt close and annoying rather than a blowout.
Watch it once with the camera on yourself, not the ball. This feels wrong and it is the whole exercise. You are watching where your car goes, not where the ball goes.
Every time you concede, rewind ten seconds and ask one question: where was I, and where should I have been? Write down the answer in four words.
Do that for one full match. If the same four words show up three or more times, you have found your leak.
Fix that one thing for the next twenty games. Only that one. A single habit replaced properly moves more MMR than five things you half changed.

The uncomfortable part

Most people who do this exercise honestly find something they had specifically told themselves was not a problem. That is not a coincidence. The leaks that survive are the ones your self-image protects, which is why an outside read on your own game is worth so much more than another tutorial.

None of this requires paying anyone. It requires one match, watched properly, with the camera pointed at the least interesting car on the field.