Sounds quite feasible!

Can instantiate multiple TES units, but need some way of choosing between them, and model some heat transfer between them? (If it is stratification)

Heat pump, not so much yet, but want to expand (eg part load efficiency, cycling etc), TES also relatively simple so far, have done most work on the loads, deriving from smart meter data etc.

I’m working on a half hourly energy model with thermal mass built in, for both heating and hot water, and working with smart tariffs and weather forecasts.. could be adapted?

It is also quite doable to estimate the energy performance of the home purely based off of smart meter use - assuming there is a relatively regular heating pattern. More work is required for unusual patterns.