This is one of the greatest pieces of music ever written in a lyric, espressivo string style, and so it is definitely a huge challenge. I feel that doing simple, slow scoring of just a few instruments is the single most difficult thing for MIDI realization. It is far easier to do a big impressive Hollywood brass and percussion showpiece for huge orchestra than this kind of music. Your performance is very good, but the violins and celli are far too stiff-sounding compared to a live group, which would be sliding all over and caressing these notes like lovers.
The first thing I would try is layering not chamber but solo strings on separate tracks that are not the same track copied, but instead are re-recorded for natural "humanized" mistiming. The solo violin for example layered very softly with the Appassionata violins is an extremely good sound. The chamber strings are not necessarily the best thing for this, though I understand how they might seem to be, as one can hear the individual tones more clearly. But somehow, they do not simply add espressivo but can change the texture into something else. But the solo instruments - especially the violin and viola, perhaps even two each of them by using the "resources" single layer looped sustain instruments, can add a lot of texture without "taking over" the way the chambers do.
The next thing is you must use the portamento AB switching in the Appassionata, or the sul in Orchestral for this music. If you don't think the portamenti are quite right it doesn't matter - there HAS to be much more sliding between notes than the very disciplined standard legato - many of which are not sul by the way - of the regular legati.
Just some suggestions. A very admirable thing to try out, and not something I would take lightly...
