That’s a lovely yet long journey that you have planned. And, it is completely covered by the Saver Day Pass.
Saver Day Pass – book earlier, travel cheaper | SBB
Except for certain PE trains, such as the Glacier Express, seat reservations in Switzerland are optional and only possible on trains of category IC, EC, RJX, ICE, and TGV.
Arriving by PE 905, you have 8 minutes to change, that’s usually well doable otherwise the connection would not be offered as such. It would be preferable if you are able to catch the IC8 at 16:48, this train starts in Brig and most passengers will board the train in Visp only. The next train from Brig at 17:18 is an Eurocity train coming from Italy and usually very well booked.
Seat reservations in trains and dining cars | SBB