EDIT: Yes, I meant physically de-solder/replace of the ROM - is that a nightmare?
What if you could find the economics of having people ship you their devices and either donating (to schools (NOT UNIVERSITY SLURPS)) or paying for their device to be up-ROM'd or e-cycled?
(My first job as a freshman in Highschool was desoldering ROMs and soldering new ROM chips into the boards of Amigas and Apple ][e's -- I had tons of these new ROMs on a strip and in tubes - but I never had any use for them... I with I had kept them.)
((Also, just to let you know the difficulty level - replacing a ROM chip on an Apple ][e etc was as simple as receiving an instruction set:
"Desolder the red guys. Resolder with these green guys."
That was the extent of my knowledge to ROMs in ][Es
I had no Idea at the time what I was doing.
You can potentially replace the OS image (confusingly, often referred to as a "ROM") on the flash storage, but unless the bootloader agrees to boot it (either due to it being signed correctly, or due to the bootloader being unlocked) you're SOL.
FWIW, I had an older Fire tablet that I had loaded Android onto and it wasn’t great. It would crash randomly and the battery life was awful. These days I have a newer Fire tablet. It’s a few years old now but I still use it all the time for reading e-books from the library, and I have a few Android apps loaded onto it that I use for working on my car and they all work fine. In other words, there’s not really any reason to reflash them, the stock OS is fine.