There are different android tv (not android TV) setop boxes available which provide an HDMI Input for PVR purposes. With little modifcation these boxes can not only be used to capture HDMI signals to an USB stick but also pipe the output into an e.g. an ffmpeg process. The HDMI signal feed into the input is also available on an HDMI output with nearly no latency. As the output is done by the android system it is scalled to the desired resolution and also 'seamless'. This makes the boxes also a very cheap scaler / seamless adapter.

• Autostart No Root is an app that can be used to run apps after boot of an android box. This e.g. can be used to start the hdmi in app
  • https://www.androiddrawer.com/25881/download-autostart-no-root-app-apk/
• SSH Helper can be used to add an ssh server to an android device
  • https://arachnoid.com/android/SSHelper/
• ADB over TCP can be used to access an android device with the android debugger bridge over network. On some device this needs to be enabled e.g. over ssh

su
setprop service.adb.tcp.port 5555
stop adbd
start adbd

There are boxes from different vendors with different chipsets available. Some boxes need extra software / modification for streaming

• MSTAR MSO9180D1R
  • Tronsmart Pavo M9 (seems to be out of production, sometimes devices are still available e.g. at aliexpress)
  • Zido X9 vendor page (not tested yet)
• Realtek RTD1295DD
  • Zidoo X8 and X9S (currently available on e.g. amazon. We don't have this devices so far but are reported to work)
  • mele v9 4k (not tested yet)
  • beelink sea i (not tested yet)
  • eweat r9 plus (not tested yet)
  • LAKE I Home Cloud TV (not tested yet)
  • Xnano X5 (available on gearbest, aliexpress. We don't have this devices so far but are reported to work)
• Armlogic
  • Fernsehfee 3.0 (currently under evaluation)