Adding more capacity

My current setup:
12 Panels in 2 strings. 5.2kWp
Gen 1 5 Hybrid inverter
8.2 kW battery.
2 x BEVs, though the Model 3 gets the most use.

I'm thinking of adding more capacity and have some thoughts that I'd welcome feedback on. I installed the system in Feb and during that month I pulled 720kW from the grid, which included car charging (about 500kW went into the cars). My current tariff (Intelligent Octopus) gives me 6 hours off-peak cheap rates.
The reasons for doing this would be to shift all usage into off-peak and to minimise export in the summer.
The easy solution:
Stick another 9.5 battery onto the inverter. Advantages- cheapest and easiest approach. Disadvantage are that from fully flat it would take over 7 hours to fully charge. Also, the maximum discharge rate would be limited by the inverter.
The most expensive solution:
Stick an AC coupled inverter into the system with a 9.5 battery. Advantages- Much higher discharge rate would cover times of heavy power draw. Two batteries charging at the same time would easily charge from flat in the 6 hour off-peak window.
Disadvantages: Increased cost and complexity. Does the GivEnergy portal play nicely with a hybrid and AC-coupled system combined?
The possibly cheaper option:
Install a third-party AC Coupled inverter and batteries. Advantage-There would be a smallish cost saving- for example 3 x 3.5kWh Pylontech batteries is about £3.3K. Disadvantages: Simply more complex management.

Am I barking up the right tree here or do I need to consider any other options?

I went another route
2nd hand DC coupled via the PV string inputs only type way. Basically it “fakes” additional PV panels into your inverter from batteries.
Was cheap (well in the grand scheme of things) with the downside being that you can’t charge them from AC but it does allow you to boost your battery only capability.

If you rummage around on TerraVolt.co.uk under Live Home I think I put a diagram of it.

Zakalwe shift all usage into off-peak

If that's your key driver, then simply adding another battery would suffice if the house load was less than say 15kWh for the duration of "no PV" hours, especially in the winter. The chances of the batteries being completely depleted before the start of the off-peak ToU tariff are low. Even on the worst days last spring, our PV sustained the house load during the day and the batteries usually started the evening with at least as much charge as they ended the previous night. Using off-peak to charge the EVs is a no-brainer and topping up your batteries to a level sufficient to get you through to the next off-peak period would work but you'd be limited to about 15kWh. We will be doing this when the days shorten and the sun is low.

Adding an AC3/battery would increase your ability to supply heavy loads such as kettle or oven but it adds a lot of complexity and cost for this benefit. Of course, if it was feasible to add sufficient PV to your system to warrant another HY5, then that could be a more robust solution, but I guess if it was, you'd have done it already.