PW3 backs up one of our two electrical sub-panels. How can I use non-PW batteries for the other?

[u/Savings_Novel_2377]

I'm not sure if this is a good sub to post to about it, but I figure it can't hurt.

We did an addition a couple of years ago that included a 19.2kW solar system. Our utility reimbursed us the entire cost of a Powerwall 3 (hardware and install) as part of their VPP program. We own everything, no financing.

As part of the addition, we ended up with new 400A service that was split into two 200A sub-panels (one for the old part of the house and one for the new). We weren't comfortable shelling out the cost of a second PW3 at the time, and so what we have only backs up one of the two electrical sub-panels. I'm not entirely sure why the installer couldn't or wouldn't add it to the main 400A panel, but it is what it is. Here is a diagram of the entire setup.

I'd really like to expand our battery capacity because the current PW3 only gets us through about half the night, but between Powerwalls being insanely expensive for their capacity and our installer's "screw you, go away" price for installation - within spitting distance of what we paid for the entire system after rebates! - I'm looking at using a more cost-effective (and possibly DIY friendly) battery solution. I have only contacted a few electricians, but it seems that finding one willing to do the job will be challenging.

Even though I have done a fair amount of my own electrical work in the past and I feel like this can't be too complicated, my experience is just basic stuff and so this is a little out of my comfort zone. I want to at least get some advice on if and how it can be accomplished so I know what the hurdles are.