I have a list of all my subreddits as RSS feeds. Did some text transforming to add the RSS links and used an OPML generator to make the file. I was not adding 600+ subreddits individually, lol.
I’ve collected loads of RSS feeds, from Congress (bills and other happenings, etc.) to the NY Times, and Science Daily with their topic feeds. GitHub has a few OPML files, though some of the feeds are out of date. Tumblr used to have a way to export your followed blogs as OPML, but that broke at some point in 2020. Mastodon has RSS feeds for every profile, but it’s a pain to collect, as their CSV export outputs the address in the wrong format for the feed.
I use Feedbro on Firefox, and QuiteRSS & RSS Owlnix on my Windows desktop. I also use Podcast Addict on my phone for my podcasts and keep a copy of the OPML file in my RSS reader as backup.
I used an Asus Android tablet (Android 7, it was OLD) with a giant SD card +& Moon+ Pro reader app. It syncs reading progress & bookmarks via Dropbox, WebDAV, or Google Drive. I moved to a Fire 10 that I added Google Play Services to. It can sync with my phone or any other Android device. I don’t bother with calibre-web as I don’t have a PC I can keep turned on 24/7 yet, so I just copy over my Calibre library to the SD card. 15k books, 512 GB SD card with ~300 GB left. Moon+ does take a bit to add new books to its database after I think 10k books.