GOLDENMATE 12V 10Ah LiFePO4 Lithium Battery(2 Packs), 5000+ Deep Cycles, IP67 Waterproof Lithium Iron Phosphate Battery, Built-in BMS, Ideal for Power Wheels, Solar, Marine, Fish Finder, Ride-on Toy

XS1500 takes 2x 12v@9ah batteries like many other 24V UPS, right? 10a discharge rate is no problem for LiFe as they are almost all 1C-2C, meaning their discharge rating is 1x (identical) or 2x (double) their capacity rating. The solution would be to get a 10ah-12ah pack of 10a is the minimum discharge you are looking for, and there are plenty of packs with the same footprint at 9ah packs. I’d recommend goldenmate since their BMS seems to work best with APC’s. I’ve installed dozens of these in various APC models without issue: https://a.co/d/06j6okbF

I just replaced 2x12v 7ah lead acid with 2x12v 10ah lifep04 on my daughters. It has worked without any issues. Stock charger works and bought a faster charger made for lifepo04 so it doesn't stop at 26.4v like the stock one. https://www.amazon.com/dp/B0DJ2HXDM1