I hadn't thought of stratifying within the tank: an excellent idea.

I had thought of having several separate tanks, both for engineering robustness in case one sprang a leak or whatever, and also so potentially I could keep one for space heating nearer 30C and for DHW nearer 60C to reduce the thermal gradients that the heat-pumps have to work against to improve system efficiency.  That would mean added complexity, and possibly needing two heat pumps, so might not be worthwhile.

Rgds