@Em0nM4stodon As some people mentioned there are mobile apps like eBoox which was okay for me. Rakuten Kobo has a 6” model which might be slightly too big but I believe meets all the other requirements. I have the larger colour model and love it.
Edit: Older Kobos have SD card readers. New ones do not but books can be drag-and-drop transferred via a USB cable. (I had no issues doing this.)
@Em0nM4stodon Not what you're asking, but found this. Maybe it can be helpful
https://comparisontabl.es/e-readers/
I know this isn't what you're asking but I retired my eReader and now have ReadEra on my phone.
Does everything you're asking and it's free.
I am frequently without internet access and have my entire library on my phone's SD card.
@Jacquetta Oh wow I am extremely loving the ReadEra app's privacy policy 🥰🔒
@Em0nM4stodon
Yes it is. A free and a paid one variant as well.
https://readera.org/
@Em0nM4stodon This one should get really close, but will lack the SD card option.
https://www.ereadersforum.com/blog/xteink-x4-pro-first-look-a-front-light-finally-arrives/
@Em0nM4stodon I like my Boox Go 7 Gen II, but it's maybe a little larger than you're after. They have a 6" that would probably be good.
They are ePaper displays but have a front light, not backlight, but it's plenty bright for me.
Ticks all the other boxes (and then some)
@Em0nM4stodon why not an actual phone? Just run it in airplane mode and stay offline. My Huawei P60 still has a memory card slot (unfortunately it's Huawei nanomemory, not microSD). AMOLED display is backlit, can run a variety of reader apps. Also my Sony Xperia V, still has microSD slot.
@hyc I thought about it, and it's what I'm doing now with an old (dying) iPhone. But then I also need to find a good app that will read easily from the sd card and not require internet to run well and add more epub books to it. I ran into this problem with my current setup and I want something stable I won't have to update or connect to the internet ever.
@Em0nM4stodon it still sounds like an old Android phone will do what you want, and there's a wide variety of reader apps. Reading from SDcard has never been an issue.
@Em0nM4stodon ive looked at this formfactor before. Id really love to see something like this that can run Graphene OS. That may be coming with the lastest walk back from Google Pixals. Who has talked about opening up to more platforms as long as the physical hardware can lock it down. That said, these two devices are worth a look: The Boox Palma 2 ($280) and Boox Palma 2 Pro ($400).
@Em0nM4stodon I am happily using the XTEINK line and rumour has it they will soon have a version with backlight. It already does all the other things you ask for.
@Em0nM4stodon not sure about affordable and it's microSD instead of SD but Boox Palma seems to fit https://shop.boox.com/products/palma
@thesquirrelfish Oh wow that seems really good, but sadly currently unavailable 😢 (and expensive) it seems. The closest to what I'm looking for I've seen so far though!
@Em0nM4stodon I like pocketbook era color (https://pocketbook.ch/en-ch/catalog/color-e-readers/pocketbook-era-color-ch). It checks all boxes, minus sd-card.
@Em0nM4stodon If you can swap the backlight for an external reading light the xteink reader has all the other features. Flashed with Crosspoint, it is fantastic. I’ve had mine for ~7 mo and love it.
@sigmasternchen I do need SD card storage though, because I want it fully offline.
@max_power Hmm no dark mode and quite expensive but that's pretty close to what I'm looking for! Maybe I'll give it a try. Thanks for the discovery!