While there is DC power the battery in a UPS isn’t used. It’s not a constant AC-DC-AC conversion, that conversion only happens when there is a power outage.

The problem is that lead-acid batteries are a really shitty technology with high self discharge rates and short overall life span (their only advantage is high burst capacity that’s good for starting motors).

I would look for 12V LiFePO4 batteries in a standard lead-acid form factor that are meant as drop in replacements. Those are somewhat more expensive (like 2-3x the price) but will likely last a quarter century or more.

I am currently using an interim solution where my solar inverter has an seperate emergeny power outlet that works even when power is cut. Sadly it doesn’t switch to battery fast enough (like half a second delay 😓) for it to work as a UPS directly. But in combination with some old regular UPS that have spend batteries that only last a few minutes it works ok. But yeah, not the most efficient option conversion wise, but solar power is free 🤷