FNFnoord·Sep 1, 2019
I've partly figured out the answer to my question.
You can self-host a backend such as Nextcloud News, TT-RSS, FreshRSS. Your client then needs to sync two-way with such a backend.
Using Newsboat (a TUI RSS reader) as standard, I'd say the following backends are applicable:
urls-source (parameters: <source>; default value: "local")
This configuration command sets the source where URLs shall be
retrieved from. By default, this is ~/.newsboat/urls.
Alternatively, you can set it to opml, which enables newsboat's
OPML online subscription mode, to ttrss which enables newsboat's
Tiny Tiny RSS support, to oldreader, which enables newsboat's The
Old Reader support, to newsblur, which enables NewsBlur support, or
feedhq for FeedHQ support, or ocnews for ownCloud News support, or
inoreader
Read the rest
for Inoreader support. Query feed specifications will be
read from the local urls file regardless of this setting. (example:
urls-source "oldreader")
For example, for NextCloud/OwnCloud (which I opted for, as I already run NextCloud, and have no clue whatsoever which one I should be otherwise using) you need to add urls-source "ocnews" in ~/.newsboat/config and furthermore configure ocnews-url, ocnews-login, and ocnews-password.
NetNewsWire looks good (Vienna looks like macOS did 15 years ago). However, it does not support nearly as many external synchronization as Newsboat does (only Feedbin, which is commercial, while I can self-host and prefer an open protocol).
As for an Android RSS reader, I haven't figured that out yet, but Nextcloud News support a lot of sync clients [1]
[1] https://github.com/nextcloud/news#Bugs