I gather you are travelling with a Swiss Travel Pass, is that correct?
In that case, your travel from Wengen to Schynige Platte will be completely free and you will not need any tickets. You can break your journey anywhere you like for as long as you like.
>> I've read somewhere on this forum that for the trains going up the mountains we'd still have to line up for tickets even with Swiss Travel passes.
That's true where the Swiss Travel Pass provides a discount rather than free travel, or for (mainly) cable cars where there are barrier gates requiring everyone to have an electronic ticket to go through them.
However, with a Swiss Travel Pass, the trip to Schynige Platte is free. As far as I can recall, there is no barrier gate, so you don't even need to get a free electronic ticket - just board with your Swiss Travel Pass.
>> So given the luggage issue it would be best to "break up" the journey , correct?
This won't be relevant, as you don't need any tickets.
>> I'd have to do the same process for the segments going to Wilderswil to Montreux.
Wilderswil to Montreux is free with the Swiss Travel Pass. You don't need any tickets and can break your journey anywhere for any length of time.
If you are travelling on the Golden Pass from Interlaken to Montreux, the trains on that route can get busy, and reservations (at additional cost) are recommended, particularly on the sector from Zweisimmen to Montreux. Making a reservation does commit you to traveling on a particular train.