And the only inverters we know will last 30 years had to have been made in the 90s …
Lead acid is much more dangerous than that, IMO, due to the potential offgassing.
This is a common misunderstanding. The lithium batteries used for off-grid and RVs are not the kind you're thinking of. They are LiFePO4, and far less susceptible to thermal runaway than, for example, an average laptop battery.