Most any UPS that has a user replaceable battery can be swapped to 12v lifepo4 batteries. they won't likely fit, but I took the two 12v lead acid batteries out of my UPS and extended the wires out the back and to a new shelf that holds two 12v lfp4 batteries. You should get 12v lifepo4s that have built in BMS to protect them from over/under charge. Mine works great and I added a lot more watt hours to it. The only thing that won't work well is the battery level percentage indicator since lfp4 don't read the same as lead acid.

I have an older (cheapo) UPS that I just replaced the battery on, which is probably over 15 years old now. It was much easier than I thought.

Coincidentally, right after I replaced the battery and left it to charge overnight, it saved my workstation from a power outage [events which aren't too common in my location] the very next day. It was quite satisfying to be able to save my work and shut down properly.

The old battery had stopped working long before replacement. So I guess the UPS was no better than a surge protector at that point.

They have them, they are called inverter-chargers. I recommend Victron or Magnum for quality with UPS grade switchover.

I keep wanting to do a Victron 3000 12v for a critical loads in my house (fridge, lights, outlets) to use with some spare 6V lead acids I have I took out of my RV when I switched it to LFP. In the future I could switch the batteries out for LFP if I wanted.

If building from scratch I would probably do a 48v rack style LFP with 48v inverter instead again probably Victron, wish they would make a rack mount version.

These exist, just look for inverter/chargers. Tripp-Lite APS750 as an example though may not be LiFePO4 compatible. I use one as a sump pump power backup.

Various Victron products as another example that are actually LiFePO4 compatible: MultiPlus, Quattro, or others depending on what capabilities you need.

Fact: All Battery powered devices should have easily user replaceable batteries