In September, the last possible train leaves at 12:48, with same-day arrival at Grimsel Passhöhe at 16:22. It is a very tight connection, though.
Having worked at Zurich Airport for 15+ years, In my opinion, to make the connection in 30 minutes is very tight to impossible. Depending on the destination you arrive (gate), and what passport you hold, the time to immigration and the immigration itself may take time. And, the baggage retrieval is not as fast as it used to be before the pandemic - due to lack of staff. It will be a gamble.
In case you can’t make the connection, there are still trains that take you as far as Meiringen. You would have to organise a private transfer from there to Grimsel Passhöhe, as I mentioned in my previous post. And, you can trust the SBB timetable, it contains all public transport services provided in Switzerland.