That looks quite a bit cheaper than the pricing I'm used to seeing, like less than half - not saying they're bad, but probably there's a DIY and risk factor involved compared to buying a more established brand like BYD or Huawei.

I'm just pointing out this is way lower than typical pricing

Maybe, but I think the premium charged by the main manufacturer's is unjustifiably high. Those battery boxes are relatively dumb: besides the cells, they just need some voltage and temperature sensors for monitoring, some power electronics for balancing, a microprocessor, and an enclosure with connectors. Unlike a grid-tied inverter, this is a really simple system, and there's no way a 500–700% price premium over the cells is reasonable.

And the nice thing is, prices will only get lower. They also have a 32.15kWh system for €3500,- which is insane.

Unfortunately, almost all three-phase [1] on-grid inverters that are on the market, especially hybrid inverters, only support batteries with much higher voltages, like 150 V or even more.

[1] Three-phase wiring in homes seems to be very rare in the US, but is extremely common in Europe.

The trick is to get an inverter with a 48V battery interface, that's not locked down to a specific manufacturer.

But you're right, most installers do high voltage batteries.