Tvheadend

I think Astra UHD will be hevc and I don't think you'll be able to play that with your hardware spec.

In theory you should see something - there was an issue with kodi that made hevc transport streams fail but other file formats work. I don't know what release version you need for the fix.

When you say "UHD from file no problem" it doesn't mean broadcast will work - the file that works may be low bitrate hevc or maybe h264 (in which case your GPU will decode it). You need something very recent GPU/CPU/APU wise to get hardware hevc decode and for broadcast bitrates most CPUs just won't be fast enough to decode in software.

I don't know what the situation is with tvheadend + UHD sat - I assume you can record and try to play the transport stream if tvheadend is setup to record like that.

Thanks for the clarification. I recorded the UHD channel 10993.75H/UHD1 by ASTRA / HD+ with tvheadend, and that went well.
Playback of the recorded UHD content gives the same result as viewing the channel with "live" TV, so it stutters.

It seems the hardware is not good enough for UHD.