qBittorrent has an option to define the WEBUI_PORT. If you want to access it on say, port 8585, set this environment variable to WEBUI_PORT=8585. Then in gluetun, continue to reference it as you do above. Hope this helps. https://registry.hub.docker.com/r/linuxserver/qbittorrent

  qbittorrent:  
    container_name: qbittorrent  
    image: linuxserver/qbittorrent:latest
    environment:
      - WEBUI_PORT=8585
    restart: unless-stopped
    network_mode: container:vpn  
    volumes:  
     - /docker/appdata/qbitorrent:/config  

If you're looking for an android variant, checkout OpenVibe. https://play.google.com/store/apps/details?id=com.plebstr.client

I've honestly been considered going the other way. I have a bunch of h.264 files that I'd like to convert to h.265. I recently heard about Tdarr, and am considering that for a future weekend project to reclaim storage space.

I think directing my friends that can to move to players that don't suck would help with the transcoding bottleneck. But some of the other responses make me feel like I'm overly concerned about it.