Loading [DRM free] books via USB using Calibre is super easy. No rooting required. Epub format works best. PDF can be so-so depending on the document.
That said, "rooting" does seem pretty easy. I use kobopatch for the sole reason of adding a button directly to the Pocket page on the home screen to save myself a "click".
What you’re looking for is Calibre ebook. And for the kobo there is a plug-in that makes any ebook a kobo ebook so you get niceties on all sideloaded ebooks that only books from the kobo store get.
You can put epubs and pdfs on it using usb, but i never tried that. I just buy a ebook on my ipad (in kobo’s store, or bol.com) and then its available after syncing.
You can definitely sideload books, and I was able to do so without loginning into an account by adding a fake user to a sqlite file on the device. I didn't see any upside to any of the tweak/custom firmware stuff I saw over just not connecting the device to the internet for my use case, but I think root access might be possible?
That said, "rooting" does seem pretty easy. I use kobopatch for the sole reason of adding a button directly to the Pocket page on the home screen to save myself a "click".
https://github.com/pgaskin/kobopatch-patches/releases