You said: "-the house lighting circuit really maxes out around 335W but what I'm looking to do is build up a 48VDC battery and use it as long as I can (via an inverter) until the software measuring it says it's too low and then throw a relay, causing the ATS to react and transfer to the other AC input."
It seems like an inverter/charger from Victron would fit the bill since it has an internal transfer switch. They have models for 230VAC and a 48VDC input. One good thing about them is that they typically can supply a surge current of about 2X their continuous current rating, which most inverters fall far short of. This might be good for LED loads that can draw higher peak initial currents to charge the capacitors of their driver circuits.
https://www.victronenergy.com/inverters-chargers
I believe that they achieve their high surge current by directly pulse width modulating the battery voltage to create a 50 or 60 Hz waveform after filtering, and then boost the voltage up to 230V in a toroidal transformer. Other inverters boost the DC voltage with a DC/DC converter first before feeding a PWM inverter, which is likely cheaper and certainly lighter weight without a 50/60 Hz transformer, but it doesn't have the surge current capacity of an inverter fed directly from a battery.
If you're from Auckland, NZ it looks like this a local Victron distributor:
https://www.lusty-blundell.co.nz/page/19/contact-us