This is what I’m planning to buy, but you know something better I’d love to take a look. https://signaturesolar.com/eg4-lifepower4-lithium-battery-48...
I have always had good experiences with signature solar. My inverters are EG4 and I am very happy with their product after trying A LOT of others. I think the primary reasons for the price difference is that you are purchasing US inventory (it's already here), it's preassembled and warrantied as a unit, they provide pretty decent support and their battery packs use 100ah cells instead of 280ah+ cells. So you are buying ~3x as many BMSs, connecting cable ($$$$) and cases for the same power.
Their rackable units are not light but can be moved by a capable person without too much fuss. A fully loaded 280ah unit in it's case is over 250lb so you really need a lift cart or such. The 280ah units and now maybe 320ah are more economical.
Building a big 14s 14kWh serial pack is really not hard, albeit those small hardware costs (bus bars) can add up. Most people don't need that much energy, probably, but these cells are just epic, maybe go 24v if you want less. 8k cycle life makes them good for a much much much longer time than most cells.
It'd be great to replace it with an LFP battery.
Will Prowse on youtube I think has some videos comparing and contrasting the different units for this purpose.
That said, something I can confirm works on that front is getting one of those power banks, plugging the UPS in, and then plugging whatever into the UPS.
I had several days of uptime on Starlink that way, running it 10 hrs or so at a time on a battery bank, the remainder on a cheap generator.