upcoming Magnificent Jewels and Noble Jewels auction in November 2019, although I don't believe there is anything relating to the Thurn & Taxis family for sale.

https://www.sothebys.com/en/articles/elisabeth-von-thurn-und-taxis-on-making-her-own-way-as-a-modern-princess

Its an interesting article and Princess Elisabeth also gave an insight into her mother's jewellery collection, which is still considerable despite the now legendary auction in the 1990s:

"And even today, Princess Gloria is protective of her impressive collection of heritage jewels: “My mother does not lend me much. It’s a big responsibility to wear them out. Whenever I have gone to parties with any bigger piece, I seem to remember fretting about not losing the pieces during the night.”