Normal tickets are for a route. So you can take any train on the route. But the normal route from Domodossola to Lausanne is not via Spiez or Bern. And it is always via Brig, as that is where the train enters Switzerland.
So normally you would have a ticket Domodossola - Lausanne via Brig, and depending on when you leave Domodossola you will be on a direct train, or may need to change train en route.
You can also buy a ticket Milano - Lausanne. That will include the bus, and you will have a reserved place on the bus, so should not have an issue getting on the one you want. That is what I would do btw. There is no need to split the tickets.