Your first rep should look the same technically as your last rep is something I learned from Charles Poliquin many years ago.
Strength is a skill. You want to learn a movement pattern and become very good at it. The more perfect reps you perform in your training, the more productive it will be. By focusing on perfect reps, there is also less chance of injury.
Some of the common errors you see once the weight starts getting heavy, or you start getting fatigued during a set, are bouncing / using momentum, cutting the range of motion shorter, or changing your movement pattern, like worming around under the bar on the bench press trying to wiggle it up, type of thing.
Bar speed may slow down on the concentric range but your technique should remain the same.
You call an end to a set when you reach technical failure, not absolute muscular failure. So, once you have to start deviating from the optimal technique, you call an end to the set there.
When you increase load, your technique should remain. Don't increase the load at the expense of your technique. This is something I see far too often. I would much rather you stay at a lighter load and use perfect technique then to increase the load and have to cheat the weight up. There are many ways to increase the overload during a training session, then to simply add more weight.
This tip is simple but it takes discipline to stick to. The rewards for doing so have paid off for myself and clients over the years so I think it is good for you to focus on. You may have to leave your ego at the door and back off on some of your current lifts for a few weeks, but it is like regressing to progress further later on. Two steps back to take 4 forward.