Here's what I've accomplished in one year - with about 12 hours of practice a week.
Bach: Prelude and Fugue in C from WTC I
Prelude and Fugue in Cm from WTC I
Prelude and Fugue in Gm from WTC I
Prelude and Fugue in Cm from WTC II
Handel: Keyboard suite in E (Harmonius Blacksmith)
Beethoven: Op. 49 #2 (polished from my older days)
Op. 49 #1 (finished 1st, working on 2nd)
Op. 79 - working on 1st movement
Grieg: Piano Sonata in Em op. 7 1st & 2nd movements (polished from my older days)
Brahms: Working on Intermezzo Op. 118 #2 in A
Schumann: Kinderszenen #1,2 7 (Traumerei)
Haydn: Sonata in D (hob XVI:37) working on 1st movement.
Poulenc: Nocturne in C (struggling with this one)
In one year you have managed to play through around 20 pieces, which is quite a rate, 2 pieces a month. No one should expect that with 2 weeks average spent on the pieces that you will have complete mastery over all tricky parts.
During the first attempt at studying new pieces we should make it a point that we identify our difficulties. The mind should be set to think when faced with physically demanding passages, Oh now here is something tricky, add it to the list, I will highlight it, experiment the physical action myself, as well as seek advice from others. So there is really no need to start thinking your technique is failing because you are making development to improve it! Much worse would be if you don't realise what you are not doing completely right.
I still find myself struggling with many of these pieces. Things fall apart. Brahms arpeggios in the left hand....The fast running passage in the development... The leaps of notes in the left hand of the Poulenc....
First of all efficient physical transfer of the body to the keyboard to me is technique. If someone looks uncomfortable their hands look twisted, rock solid and rushing all over the place, this looks like bad technique to me unless the pianist insists that their hands feel absolutely relaxed. If it looks all effortless and they do it all automatically this to me looks like good technique.
When you are faced with anything physically demanding on the piano you should strive for relaxation, an "effortless" touch. You should never be tense, or or feel uncomfortable, no matter what speed, what volume or notes. This sense of control is what we always aspire for.
There is ALWAYS that one note which causes our difficulty. You have to pin point which note that is and practice it away with repetition AND with the aim to improve efficiency and an more effortless touch. With repetitive investigation to these difficult points you eventually learn to spot them in the future and utilise technique to deal with it immediately.
Sometimes we do not understand what it is exactly we have to do to improve the efficiency. There are really a huge number of factors contributing to an efficient touch to the piano which are better explained through action at the piano than catgorising them in text.
I personally think these are the most important points when considering efficiency in your transfer to the keyboard. Knowing where the Centre of hand is (which finger(s) provide a balance for the hand during a passage), maintaining a lazy hand (a hand which doesn't like maintaining an expanded hand posture or a contracted hand posture but which constantly tends towards a relaxed hand posture i.e: Rh 1 on E, 5 on B, middle fingers on black notes, Lh 5 on F, 1 on C, middle fingers on black) and Effective Movements of the hand/body to produce desired sounds are perhaps the compasses that command efficiency.
A lot of the times we have to be shown what to do by more experienced pianists to improve our technique. But so long we experiment with the aim to increase the effortless touch of a passage, we will be looking in the right direction. A lot of people think that there are passages in music which require us to get tense, to use brute force, get uncomfrotable, they think this is the challenge and what needs to be controlled, this is really not the case, not for a refined technique.
If arpeggios are causing you problems you have to consider the structure of the arpeggio closer. What shape is it, how does it effect the natural shape of the hand, where is the centre of the hand while playing certain parts of the arpeggio that is which finger provides balance to the hand during the passage. If the hand has to change shape perhaps we can minimise the change by carrying our hand across the keyboard instead of stretching with our fingers so that they all physically wait as close as they can above their notes.
By moving our hand across the keyboard we may make the distanced required for the fingers to strike the note lessen. We then have to question how do we use the weight of the hand to play the notes of the arpeggio isntead of individual fingers. Of course this is a generalisation there may be instances where each note of an arpeggio has to be individually drawn out, it is all a matter of context and knowing what sound needs to be produced.
The Bach WTC you have picked up is constantly asking the question as to which finger(s) balance the hand while playing a phrase of notes. We are also have to consider when the hand has to move and when it doesn't, when it holds its position and maintains balance about a particular finger. So we can't be moving when we should be at rest, and we shouldn't move abruptly when we have to move, movements should be always gradual, we should "feel" as if we are always coming from above when playing notes never throwing our hands haphazardly sideways to a note.
Technique is incredibly difficult to explain in words for any disipline. No text will teach you how to swing a golf club, neither will any text teach you how to play piano. It comes from trial and error and improving our intelligence to think how to physicaly control our physical action required to play our instrument with an effortless touch. That comes from discussion and watching other great pianists play in front of you and of course advice from experienced teachers who can tutor you one on one.
But your effort for one year is really amazing after such a long absence from the piano, and it will only do your techinque good. Increasing our memorised repetiore automatically improves our ability at the instrument.