I would say that there is no such thing as firmware targeted for regular consumers. Regular consumers do not know what firmware is.

I am still disappointed that there is no kindle sized hardware that this can run on, but I think that has to do with manufacturers rather than firmware developers.

I mean, a) regular consumers are just gonna buy a Kindle, do nothing with its software, and only buy books from Amazon. But also b) the way a lot of these hobby things expand is that if they get some market traction, someone starts making and selling pre-assembled, retail-friendly versions of the thing. Look at wacky keyboards, for instance: Used to be that you had to solder your own weird ergo split, now there are companies like ZSA and Keebio that'll make really nice versions as finished products.