I’ve been gifted a Sony PRS-T3 over a decade ago. I’ve recently gotten into reading again and used it to read a manhwa/webtoon/web novel (or whatever the Korean ones are called) and most recently a light novel.
It’s functional and perhaps even decent (especially given its age) but my main gripes with it are:

• Size: It’s much too small to fit an entire manga page with readable text, so you need to use hacks like kcc which is suboptimal. I’d like the display to be the size of a typical manga or slightly larger.
• Lack of customisation: It has this ugly indented paragraph style in books which I don’t like and the selection of fonts aswell as font rendering isn’t great.
• Artifacts in images: When anything more complex than text is on display (and even with text it’s subtly noticeable), you always see ghosts of the previous image. This is perhaps the most critical flaw for the purpose of reading Manga. Image quality in pictures isn’t great to begin with either.
• Slow: Page turning is fast enough but doing anything else it turns into a slog. Switching between “books” (the manhwa had each chapter as a separate book) was annoying to say the least.
• Bad UI: It’s just generally poorly organised and common things required way too many interactions (which, mind you, are slow).
• No light: I appreciate not requiring a light but I’d sometimes like to have the option.
• Ergonomics: It’s light but not very comfortable to hold. I think I’ve seen readers that have a thicker end on one side so that you can better hold onto it? I’d appreciate advice here.

It’s also showing its age; I had to tape the lid already as the material started to disintegrate.

I did very much appreciate how simple it is though. Open the lid, it immediately turns on, (I enter my PIN) and I can continue to read my book where I left off. Just like a real book but more convenient. I’d like to retain that property.
Battery life is also still great, even after all these years. I can close the lid and leave it sitting around for weeks and return to it with barely any battery drained. Again like a book where I don’t have to worry about any battery charge either.
It’s also quite light which I like, though a little bulky but totally acceptable.

Deal breakers:

• Enshittification: If the primary purpose of the reader is to sell books rather than read them, I don’t want it.
• Espionage: I don’t want Google, Amazon or anyone else spying on when I read what books. I’m probably going to have its networking off anyways but I don’t want anyone spying on me offline either.
• Gesture-only page navigation. Physical buttons please.
• Ads of any kind.
• Any power/data connector other than USB-C

I don’t care for DRM. I’ll be loading epubs onto the reader from another machine.

I don’t think I need colour. I mean, it’d be nice I guess (especially for manhwa, those appear to frequently be coloured?) but if that compromises on greyscale or text clarity, no thank you. I also don’t know whether e-ink can reproduce colour accurately enough that it’s even an upgrade over greyscale and doesn’t just look ugly.

FOSS firmware would be amazing but my research suggests that’s not really a thing? I’d settle for a decently customisable proprietary firmware as long as it doesn’t suck donkey balls or needs to be connected to the internet.

I don’t need to draw on it.

Price is secondary but I don’t like wasting money either.

I’m in Germany/EU.

I don’t have a single clue about the e-reader market. I’d appreciate any advice on what I want and, more importantly, don’t want given the constraints and desires I described.