I mean, it's not Jerboa, but I did (or tried to do) all of that with Tesseract. If you "install" it as a web app (e.g. "add to home screen" from the browser), it's pretty much indistinguishable from an app app. The only "app" thing it doesn't do is system notifications. I'm not sure that's something I want to deal with, though. I have the code mostly in place to support it, but I don't know if I want to deal with trying to keep it running in the background to receive them.

For the features you mentioned:

• Custom Feeds: I had that for a while, and it worked, but the Lemmy devs removed the ranking metrics from the API which broke sorting except for date and score before I could get the feature fine tuned. May or may not try to work around that and bring that feature back. For now, the community groups (formerly custom feeds) are just used for organizing communities (which is still useful).
• Content Previews: Pretty much any outside link is previewable; it'll pull the metadata, give you the option to try to load it in an iframe in the app (not all sites allow that), and if the link is to a supported media (YouTube, Loops, PeerTube, Spotify, etc), it will render that in the preview modal. e.g if someone comments a bare YouTube link, clicking it will let you watch the video as an embed right there.
• Video player: Every embeddable media format I can support I try to. Currently:
  • YouTube/Piped/Invidious
  • PeerTube
  • Loops
  • Spotify
  • Bandcamp
  • Odysee
  • Soundcloud
  • DailyMotion
  • song.link
  • Vimeo
  • Imgur GIFs and videos
  • Direct videos (mp4, ogv, webm) and audio (mp3, ogg, aac)