Neat arrangement! Very good string writing in general. Three cellos against two basses will provide excellent balance and you've used the 1st cello's highest register well.
As a cellist I have four critiques -
1. Ab major is very readable, but Cb made me go "eek." Just as clarinets and trumpets find flat keys easier to read, strings play easier (and sound better imho) in sharp keys. This is an all string piece so no reason to play in a key that's more appropriate to a band piece.
2. String scordatura mid-piece is highly inadvisable. Just don't
3. Tremolos are easier and sound better than rapid string crossings, on bass, and to a certain extent in low cello parts as well.
4. C-extended basses correspond approximately to alto flutes in terms of the Don't Write For One Unless You Know It's Available factor. They're not rare per se, but uncommon outside professional orchestras.
The solution to all your problems is to transpose the piece up. I recommend Ab->D and Cb->F. This will make the piece brighter, infinitely easy and enjoyable for 4 of the players, and slightly challenging for the 1st cellist, but certainly still easier than reading Cb would have been
Anyway the highest note I see is a thumb position D - a good cellist can go far above that.