ReadyNAS RNDU2000 fried after power supply failure?

Was your Ultra running OS4.2.x or had you converted it to 6.x?  r-Linux will work for OS4.2.x, but not 6.x.  And there is always the chance that the volume was corrupted as well.

It sounds like the flash memory in the unit is corrupt, or perhaps fried.  You can try a USB recovery to see if that fixes it.

As for new ReadyNAS, the supply has been pretty much zero since a little after the COVID lock-downs.  Many businesses bought up what was available to assist in work-from-home solutions.  Whether it's supply issues or a choice of which products to prioritize during COVID or whether Netgear is planning to discontinue the ReadyNAS, they have not said.  But the used market is about all there is right now if you plan to stick with ReadyNAS.

If your NAS was still running OS4.2.x, then you can't just move the drives from the old to a new OS6 NAS and have all the data intact, BTW.  There is a method of using an OS6 NAS to recover data from OS4.2.x, but you have to move it to an intermediate location, and Netgear will give you no help in that regard with a used NAS.