Some of the power stations from Ecoflow/Anker/Bluetti are competitive in terms of price and capacity while still having a fast enough switchover for UPS purposes.

They tend to have features that may not be necessary for a UPS (eg solar or DC input), while lacking some features that are more common on UPS (eg companion app to turn your computer off when UPS gets low, although you might be able to rig your own solution)