Already started :
https://www.tomshardware.com/news/asrock-z490-motherboard-intel-atx12vo-10-pinIf youre willing to make tryouts of your "new" ups design and put time, efforts and maybe more than you're willing to spend go for it.
You have efficiency loss to convert a dc source to smps / ups and convert it again to the needed 3v, 5v, 12v and -12v, for the motherboard needs, and the rest of the computer case hardware like vdo card(s), ssd, sata hdd...
Each voltage you need will have a loss somewhere, and finally you'll need lots and lots of amps ... and some protections ??
I have a 1k watt psu for a computer with 12 hard drives, my 1.5kw apc ups last 10 minutes with the original 24vdc internals batteries, 20 to 25 min with the added external pack. I dont know your computer power requirements ?
My 1kw corsair psu.
3.3v is 25 amps
5v is 25 amps
12 is 83 amps
-12v is 300 milli amp
5v standby 3 amps
A big total of 137 amps

Some of my friends are using deep discharge cells, they have put an seconday charger on them, when the charger operate it select a battery pack and open the battery circuit to charge them outisde the ups system, once the charge is complete they are put back in the circuit, while the other pack begin a charge. But they are 3kw or more ups ... in rack mount enclosures.
I do understand the 2-3 years change, my old / died ups had a charge problem, it busted the batteries after 2 years, but i must say it saved my ass so many times ...