You can get a Kobo Reader and disable internet access to it so it never connects to a server. You can then plug it in to your computer and it shows up as a mass storage device. Then just drop PDFs, ePubs in.

I never liked Calibre, it's weirdly shoddy software, slow as a dog, and the worst UX i've ever seen in a popular app - so I needed something I could just drop my files into.

I do the same thing with a kindle. I've never had it connected to the internet or used any amazon services with it. All my books were just moved over via usb.

The weird thing is how huge Calibre is considering, I'd wager, 90% of people (myself included) just use it to convert books and never touch 1/100th of the tools and functionality in it, not touching on the fact that it's not a shining example of intuitive software. But once you have it setup, using it as a middleman is pretty straightforward and easy.

Is there a simpler conversion tool that does as good of a job? I've literally not looked in a decade plus.

I did this with a pocketbook. I wish I could recommend it strongly, but in fact the USB port is extremely finicky (often can't charge, can't get Calibre to detect it). As it is I can only recommend it as the cheapest ebook reader that's comfortable to read from.