Used Supermicro 846 and 847’s are a bit cheaper and more bays, and easy to modify to be fairly quiet.

Yes. The Cyberpower’s tend to use a 9AH battery in the same form factor that’s normally a 7AH, but it’s just a higher capacity. Check what physical size the UPS uses and go from there. Keep in mind you can have different battery sizes for the same capacity UPS. APC used to make a 280, 420 and 650 that all used the same batteries, and a 650 that used the next size up. All appeared identical except for the cutout in the bottom where the battery was loaded in.

Added bonus is you’ll save some electricity by using 208 or 240V as it’s more efficient.

You may need to pick up some C14 to Nema 15 cables for anything that’s using a standard 120V plug. I suggest labelling the plug as 208V or 240V so you don’t accidentally plug in something that really is only 120V capable.