playing devil's advocate here:
There's little or no money to be made from selling better e-reader hardware. hence Sony's decision to quit, and Nook's continued dithering, leaving only Kobo & Amazon as real players. The Aura HD was/is a trial run / limited edition & the best selling devices are probably the lower sec ones with far fewer options.
Kobo's profits come from selling you e-books from their own book stores; so as long & those display reasonably well, why should they care about your poor-side-loaded experience. ( we know they don't, actually, because of the kepub vs epub, missing reader stats etc debates )
So unless the reader community develop & maintain some open source custom firmware, ( and the device makers don't want that so they make their devices hard to root - you aint going to get what you want. )
also, we'll soon be intro the usual circle of blame-> its the authors/publishers fault for poor CSS, no it's the reenderer's fault, no it's the lack of proper standards... take your pick.
then there's the argument that if the author only wants his/her book displayed in , say, Charis, should that be enforceable, or should your end-user choices have precedence ?
I read a lot of library loans & many of those have locked fonts and/or line space settings
PS - much as I like hi-spec gadgets, there was a lot to be said in favour of my old Sony T2- just open the box, charge it, choose small , medium or large font - off you go.... no stressed out search for how/why all the extra bells & whistles do & don't do what you'd expect
. ( & no forced sign up to store, no forced
install desktop software or you can't continue, no forced:
here's a firmware upgrade whether you want it or not...