There are maintenance works scheduled between Interlaken Ost and Spiez between the 14th and the 19th of June. You are to expect modified timetables, cancelled trains and replacement busses. The online timetable will be modified, but as far as I can tell it has not yet been modified. Within Switzerland, there are no other disruptions announced as of yet and probably there are no more to come. For Italy, there are replacement busses between Iselle di Trasquera and Domodossola. One of the routes between Domodossola and Milano is also closed, with not yet announced replacement options. Those works cover both the 10th and 15th of June.
Given that you ‘NOT’ want to be on a bus, your best option is probably to completely change plans and not to travel via Spiez and Brig. Instead, when travelling via Meiringen - Luzern - Arth-Goldau - Bellionzona, you will have no announced works both in Switzerland and in Italy. But here as well, I think a definitive confirmation for that will be impossible to give. Especially in Italy, the train staff is on strike relatively often compared to Switzerland and this is unpredictable so far in advance. And then there is the unforeseeable events like a tree as part of a strom falling onto the tracks or anything wild in this direction. It is rare, but nobody can guarantee it doesn’t happen.