Hello WKru4830
Good day.
There is no need to reserve seats for this trip. No local would ever reserve a seat when doing this trip.
For some trains, seat reservations are not possible for operational reasons.
There is no possibility to select a specific seat. The seat reservation system is very rudimentary and does not offer a lot of options. This is because we, the Swiss, do rarely make use of seat reservations.
Just get on board the train and have a ride. Alternatively, you could take a train that operates Spiez - Zürich HB on a direct route, without change at Bern.
Kind regards,
Roland