Your pricing information is a bit out of date; with current Government incentives a 10kW array and 50KWh batteries can be gotten for less than $10K.
True, general feed-in rates have tanked due to a glut in solar panels, but certainly providers do still offer generous rates for shoulder periods (around dawn and dusk) where you could potentially discharge part of your battery into the network (I think?).
But the real savings from having the batteries will eliminate your need to use power during evening peak times, eliminating the need for buying petrol if/when switching to an EV and having the ability to buffer energy from a rare sunny day in winter into free energy for the next 2-3 overcast days.
Depending on how aggressively you monitor and take advantage of market offers, and electrify your home - the pay-down period can be as low as a few years.
40-50kWh given the current incentives (which are changing in May) only costs only a few hundred dollars more than ~15kWh, and provides owners with more flexibility in terms of EV charging (especially if the car is used for daily commuting and would not be able to directly take advantage of solar), and discharging back into the network at optimal times to recoup the system cost that much faster.
But otherwise, I do agree with you - our peak usage is less than 16kWh/day (2 WFH adults, 2 grandparents and 1 child); while our solar exports usually triple that amount in summer and we usually net-0 in winter (ie. the amount we export matches what we buy in over those ~3 months).
We don’t have a battery installed yet - but we are very seriously considering it, hence I have the figures available off the top of my head.