What i did is, jailbreak [1] my Kindle Oasis, and install KOreader [2].

This gives you full access to upload whatever ebook you want (SSH, WebDav, Syncthing, ...) and it can fetch RSS feeds (i use it with FreshRSS.)

PS. The (very old) Kindle Oasis is still the best device there is to read on in my opion. Which is crazy, since it was released from 2016-2019...

It has 2 phyisical buttons to turn the page, and an ambient light sensor to auto adjust the brightness, and a 300 PPI display.

I'm still "waiting" for a better / equal device to be released.

[1] https://kindlemodding.org/kindle-models.html

[2] https://github.com/koreader/koreader

Not only that but KOReader has native support for OPDS feeds, which the OPs read-it-later service Readeck also natively supports.

I have been using it like this for a while and it is absolutely bliss to be able to view a catalogue of my inboxed articles on my kindle, with annotation (exports of which are also supported in KOReader).

I missed my oasis dearly but I couldn’t wait anymore and got a Kobo Libre (not sure the exact model, th color one); it’s pretty much as good, only thing i miss is the dual battery system.

Koreader is well supported and has all the features you mention.