I see. I may try to do something similar but towards Unbound on my OPNSense router, if that’s possible.
I suspect DoT and DoH still go through, though? I mean you can always block the port 853 for DoT, but DoH is another story.
For your links (I believe it works with files too), check out Heimdall. It’s not offline, but it’s FOSS and selfhosted.
Kobo devices are easy to install KOReader on.
You should also ask yourself what kind of books you want to read. Black and white comic books, for instance, can be read on an e-reader, as long as the screen is big enough.
The US Army would disagree, and I’m certain they know better than you. They literally use CoD in their recruitment campaigns.
Project management: Projeqtor
CRM: Dolibarr
Both: Odoo