I'm trying to book a Sparpreis train ticket from Germany to Switzerland using both a BahnCard (to get a discount on the German part) and a Halbtax card (to get a discount on the Swiss part). At bahn.de, I can choose either a Bahncard or Halbtax, but can't seem to choose both. At sbb.ch, I can choose both, but available connections are limited; some connections are not bookable at all, whereas others show only the Flexpreis and seat reservations are not available. Although Railplus still exists until 2023-12-31, it is not valid in Switzerland (see also this question). I could split my ticket in Basel, but this might impact my rights in case the train from Germany is late.
Is there a way to buy a ticket from Germany to Switzerland while applying both Bahncard, Halbtax, and still buy a Sparpreis?