Highly recommend Tripplite
I got me one of these used (with good batteries) for $100
https://www.newegg.com/tripp-lite-smart1500lcd-5-15r/p/N82E16842111052
They have an external connector in the back for adding external batteries and the menu to do so is analog.
I added a golf cart battery to mine tripping the capacity for very very cheap.
Look in the used market.
If you want something better, Eaton is the go to, look for used deals in marketplace / craigslist. I found someone selling Used industrial Eaton UPSs with extended batteries for 150$ the set
My body is ready