Biggest speed difference I’ve seen for my Kobo was adding a faster class 10 Micro SD. The one they ship with is fairly lousy. Plus now I get 128 GB instead of 8 GB.

When you say you turn on wifi, do you change the wifi password on access point after it? Some TVs have been known to remember and use wifi credentials even when disabled, iirc. I never let any device on my network unless it is properly blocked from phoning home.

You don't even need Calibre. Just connect to your computer's USB port, it'll show up as a mass storage; copy your ebooks over and you're done.