I used wallabag and liked it. I stopped when I had to move my server and backed up the data. I installed wallabag again and the backup would not work with the latest version. I had exported it in various ways as well, just in case one didn't work well but none worked. So that left a bad taste in my mouth. This might have coincidentally happened right in a database upgrade or something inbetween the 2 versions, but the fact that my backup could not be used frustrated me. Still looking for an alternative and am considering readdeck like you. I tried it a while back and did not see a way to export data at that time (I think it could have been done from command line though?). Been over a year since then so maybe that has changed. Right now I am using Shaarli for bookmarks and it works. It doesn't stick for me like wallabag did though.

I just set up Readeck a few weeks ago, and I've been liking it. Very minimalist, utilitarian. One feature I'd like that isn't included is the ability to add specific labels or collections to the sidebar, but that's my only quibble so far.

It has an official browser extension for adding urls to it, but if you can't or don't want to use that, it has a nice api. I use the api to add bookmarks from my phone using a termux-url-opener script, which is as easy as the extension - just hit the "share" button and select termux, and it does the rest.

I have no experience with Wallabag, but I have been pretty happy with Readeck. Skimming through Wallabag's documentation, I would say they are pretty similar, while both have unique features. For example, Wallabag has annotations (you can only highlight in Readeck), and Android and iOS apps; whereas Readeck can export collections to eBooks, has RSS feeds for pretty much anything (all articles, unread, archives, collections, etc.) and its browser extension allows to only save part of a page (by selecting it first) and to directly send the page content to your instance (which is useful when saving paywalled content)