How to connect old solar to new solar and battery?

I have a solar system installed in 2015 consisting of 12x280W panels and a string inverter. The solar company that installed has folded but the inverter still has 3 years of warranty AFAIK.

I’m getting quotes for 6kW more panels and ~20kWh of battery. I have quotes for an Enphase system with micro inverters and Enphase batteries, SolarEdge with a string inverter and SolarEdge batteries, and SolarEdge inverter with SonnenCore batteries.

I’m being told that installers don’t want to connect the old solar to the new system, but I’d really like the old panels to contribute to charging the batteries.

What are the realistic options of making that happen? Can the AC-coupled battery systems accept AC power from the old inverter alongside the new one? For the DC-coupled Solar Edge system, is it possible to oversize the Solar Edge inverter and (after install, pay a solar company to) connect the old and discard the old inverter? What other options are there?