I listened to the piece several times, also using headphones, and I share many of the impressions already expressed.
First of all, I tell you that even Berg, who was from that period, wrote serious "gentleness". Therefore, it does not represent a problem because you are not Schoenberg; you live today and not a century ago... so take a series and do whatever you want with it. You say that
"Besides, I have understood that when the series is good... half the work is done."
Not only the choice of notes (and intervals, especially. Implicitly) is important but, as Thallo rightly says, also its rhythmic articulation... which at least must arrive in its development. But even here, when is a winter day truly defined and articulated?
To push you further, the first useful suggestion that came to mind has already been spent by Thallo (who always "steals" the best answers from me)... anyway, it is precisely to chew on good Messiaen (Thallo, are we related?) who, at a timbral level, is one of the greatest advocates of the last century, if not the foremost exponent.
Apropos of the performers, I was most struck by the viola... even if by definition everyone takes it out on her. I usually trust good performers and they often give rise to ideas and good desires, so the real question is... why didn't she comply with the fff. And yet, it doesn't seem like a first-rate issue to me.
I confirm that my greatest desire was to truly hear silence when it was written ... a pity about the audio file, but with a good imagination
Anyway, I consider it a good piece, especially based on the response you were able to give to the difficulties you set yourself to overcome.
Making a series current, with an interesting but difficult instrumentation and pushing into timbral research.
Keep in mind that it is harder to make 2 instruments sound than an entire orchestra, so if I were you, I would generally be satisfied.
I like the sound space which I would define as "desolate", a place to which I am very fond and if I were a man from the first half of the 1950s I would abuse it... I mean exactly this indication, although I notice there is a rediscovery of the fashion of using descriptive terms in tempo indications. So, to this, I would write...
My compliments and until the next work.
Frank
PS
Regarding the discussion on rules, I share what has been written by almost everyone; it is also true that we are talking about rules of taste, the taste of a certain period... over time you will build your own dictionary which will in turn be driven by taste, your taste, and you will create a little "rule-abiding" or "regulated" corner where you will feel at home and "reassured""