I really tried to use a 6'' kindle for that, but it just doesn't work. I tried it with .pdf, but the screen is just to small and scrolling is not really comfortable. When I tried to convert them to .mobi with calibre, all formulas just looked nasty.
Today, I read them on my 14'' ThinkPad with Redshift installed. It does its job well, but it isn't as handy as a Kindle would be.
Right now, I also try to keep Google as far away from me as I can. Unfortunately, I don't own a domain myself. Have you tried out ProtonMail yourself? If so, do you think that it is a good alternative to a self-hosted server like Fastmail?