I'd definitely go the Nvidia Shield TV (2019) route, I use one for exactly that. It is a tiny box that runs Android TV (which includes Chromecast functionality plus the ability to run apps local) and even the original 2015 model is still getting updates. For control it comes with a hardware remote or you can use your phone or you can cast to it. Supports 4K HDR10, Dolby Vision, and Dolby Atmos as well as full DRM support for Netflix/Amazon/Hulu/etc allowing it to actually receive the 4k versions (or you can just use the Plex app if that's your thing or VLC for local files).

That extractor will passthrough whatever the output is and break out the audio to SPDIF for your soundbar.

Overall it's on the more powerful side and has full hardware acceleration so weird latency issues but you'll want to match the audio offset in the Android TV settings to whatever the display latency of your monitor of choice is to get the audio/video timing to perfectly match (supports both positive and negative offsets in case your soundbar has its own latency problems).