how to bias BJT in large current?

[prcken]

Vcasc is not independent and it has to be lower by at least Vce(sat) from VDD to prevent the bipolar transistor to entry into saturation.Also Vcasc defines also Vds of diff pair and Ve of bipolar pair.You should find a so proper Vcasc that satisfy those constraints.Increasing or decreasing Vcasc will not practically change the tail current but it will absolutely change the max. voltage swing.

Yes, boss.
I think Vcas should be less than VDD-Itail*RL (when all the tail current is steered to the side). As you said, also need to watch the voltage at the collect of the diff-pair.