The piano part is too much repetitive, you should really try to use more different chords (not harmonies) or piano progress (counterpoints, thematic material, more varied rhythm, motivic variation and development etc...) As far as you address the use of high register: actually, it's not the highest register at all - cello can relatively confortably play up to d6. It is accesible to most players with about 6-8 years of cello playing.