Our helpful perspective on improvement naturally must include our brain and body. Understanding our strengths and limitations inform expectations and approach in practice.
Brain and body - a team
The culture of golf has a tendency of separating the two. The mental game often gets pigeon-holed as something "I need to work on once I've got my swing sorted". Conversely, many golfers treat their swing like some piece of machinery, detached from a brain. This frequently interferes with some of our natural learning processes and improvement is elusive.
The reality obviously, is that the brain and body work in tandem, simultaneously. Our brain perceives a task that needs attempting (e.g. hole this 6 foot putt), and signals are sent to our body to prepare. At the same time feedback signals from our body tell the brain how we are doing, and adjustments can be made. It's a continuous loop that give us huge problem-solving potential (if we don't obstruct the natural processes).
Our swing is in our head
Our swing is a partly a product of natural biomechanics, but the motor pattern that encodes the shape and sequence of our movement resides in the brain (in structures like the Primary Motor Cortex). Changing your swing might be expressed physically, but it is encoded in your brain. You are literally re-wiring yourself when you improve.
The degrees of freedom problem
One of the features of the human body we need to be reconciled with, is how un-machine like we are. The term "degrees of freedom" was coined to capture the staggering amount of options our body gives us. The sheer number of bones, ligaments and joints means we can turn our hand to a multitude of tasks. It's part of our evolutionary success as a species.
The downside to this is consistency of movement. Such are the degrees of freedom in our body (even those of us with stiff backs!), that:
It is physically impossible to make a motion as complicated as a golf swing exactly the same repeatedly. No golfer has ever done it.
No amount of balls hit will overcome this. Excess tension is often a sign the golfer is subconsciously trying to lock out the joints and restrict variability. This is pointless because the cost is speed, fluidity, good sequence etc.
The upside of this variability is adaptability. A robot with a fixed motion might beat us on a driving range, but would be much less of an opponent as soon as it had a side slope or a lie in the semi rough to cope with.
Redefining consistency
Given the above, attempts to "lock in" a new swing are to some extent, futile. So I would like to redefine consistency for you. Golf benefits from effective and predictable shots, so consistent outcomes are certainly the goal. However if we accept that it is impossible to move in exactly the same way each time (bad variability), then we need to learn its antidote, good variability.
Good variability
All consistently good ball strikers have honed an ability to make subtle corrections within their swing to deliver the club as intended, in spite of an iffy backswing. These are not consciously controlled, rather "self-organised" movements which are a product of a certain kind of practice.