Honestly man, I think you just need to read around these forums. You have maybe too many wants that are very particular and sometimes contradictory.
Possibly your best bet is to buy one of those readers that runs Android. But I have no experience with those, so I can't recommend one.
I do think a Kobo might fit many of your needs. A Libra (7" screen) or a Forma (8" screen). You have to put in an email address to register it, but you can make up an email address if you'd like.
Here's a list of your features that Kobo meets:
Nice to have's:
*No WIFI - All readers nowadays will have WiFi. It can be turned off.
*Open source/hackable/changeable OS - Kobo uses Linux and is hackable
*Lots of settings is nice unless the device is very minimalistic and balanced - Kobo has lots of settings and more can be added via patches/hacks
*I prefer systems that have their firmware updated, instead of "released and forgot" by the manufacturer. If it is possible to install a custom OS then of course this doesn't apply. - Kobo is very good about releasing firmware updates for very old devices. And you can sideload the newer firmware so you don't have to turn on WiFi and let all the demons in
* I would like to use the device in paranoid mode. As in, I wish to never use WIFI, if it's there. So no downloading/buying from stores or libraries. Just simple PC-to-ereader transfers strongly prefered. - I just don't get the paranoid mindset. But anyway, you need to register your device and you can then turn WiFi off forever. Kobo works very well with Calibre/PC-to-ereader transfers
*nice and cozy orange light is <3 - The Libra and Forma both have orange lights
*Doesn't have to be comfortable to use. I don't think I will care much about button placement or lack of buttons or whatever. - Both the Libra and Forma have buttons.
*Good battery/Good battery time - I think all ereaders have good battery life
*Removable battery is probably too good to be true? I would like to be able to change it in 3 years or so. - Good luck. I don't know of any current gen ereader with a replaceable battery. Maybe they exist.
*Basic PDF support is big plus. Got some journals to read. No need for anything special in this regard. - I'm glad you say basic, because if you really wanna read PDFs, get a tablet. All readers claim PDF support, but basic is a fair description.
*I want to feel and know that I own the device after purchasing it and forget who made it - I honestly don't know what that means precisely, but you can hide/remove the Kobo store link from the home page
*Preferably highly power efficient hardware. Don't care about speed at all. - I think all e-ink readers are pretty power efficient. The ones that run Android less so than say, a Kobo, Kindle or Nook
*No bluetooth or SIM capability, if those are even 'a thing'. - I don't believe the Kobos I mentioned have Bluetooth? Someone can correct me if I'm wrong.
*Preferably no loudspeakers, no SD/other card slots. - I don't understand why you WOULDN'T want SD card slots. But they aren't available on the Kobos anyway.
Must have's:
*Not Amazon - Kobo isn't Amazon
*Not bound to a single ecosystem. Shouldn't demand registration, or require internet connection. No ads. The device should wish to serve me, not limit me. Shouldn't be intrusive in any way. - If you're sideloading all your books, I'm not sure what your point is here? Kobo has a store, like everyone else does. There are no ads on a Kobo, unless you buy from Walmart and if you do, that can be turned off easily.
*Good build quality. I want it to last for years. I understand even cheapest plastic sheit could last a decade, but still would love to have something made with care and quality components. Preferably sturdy built but not necesserily. - You might want to read up on the Forma and Libra and their light bar thingie. I don't have one, so I can't say
*6-9 inches. So I suppose medium/standard size more or less. Portability is not a huge priority. Doesn't have to fit in the pocket. - As mentioned, the Libra is 7" and the Forma is 8"
*waterproof/dustproof - The Libra and Forma are both waterproof. Not so sure about dustproof.