You're probably right that just engineering a specific solution or ordering them from a supplier that makes such things is probably less hassle and possibly cheaper in the long run.
I'm currently doing an EV conversion. I'm amazed that you can buy a box that's about ten or fifteen pounds or so for a couple thousand dollars that can handle about 90 kilowatts of power (in short bursts), converting around 100-200 volts to AC power to drive a motor. That's probably enough to power a whole residential block by itself, and yet it's not even a very powerful controller compared to what you find in a high-end OEM EV. I'd expect anyone building a large scale grid backup battery storage system to have access to inverter parts at least that good and at least that cheap, and if they don't they could afford to design it themselves.
[1] https://ai.googleblog.com/2016/02/and-winner-of-1-million-li...
but there's no general market atm for such inverters. so building lots & lots of lower voltage packs & using low voltage solar targeting modules, which are everywhere, works out.
but it's for sure not as good & makes less sense.
repurposing motor controllers is an interesting notion. but I'd probably rather diy than try to circuit bend that. big awesome mosfets &c are cheap. motor controllers have to deal with inductive loads (big coils) so they tend to have slow slopes. which matches up ok for low 60Hz. but it's still be an effort to repurpose & reuse. just redoing the inverter seems like the win. don't even try to find out what design parameters don't line up between motor drive & inverter drive, don't spend the time, for something that won't work as well as having the right thing.