It's been a couple of months and my ET10000 is working well. It's not 4K but I am very satisfied with my setup and looking forward to exploring its full capabilities in the future. I write only to register my appreciation for the guidance/help from so many members of this forum.
In case it's helpful to others considering this old but still excellent sat box, I was eventually able to find a very reasonably-priced new DVB-S2 satellite tuner from a seller ("fettsbook") on the German Ebay site. The tuner is the same as that used in the XTrend ET8000. There was a solitary seller on Amazon but their price was over twice that of the Ebay seller even including postage.
The ET10000 box and tuner combination does not provide RF outputs but this is easily done using an inexpensive standalone RF out box as advised earlier in this thread.
Thanks all.