Gab1210 Where possible I prefer to book tickets as we are a group of five adults and it can be tricky to secure seats.
In Switzerland tickets do not come with a seat. This is like travelling on the train in Sydney or Melbourne. You need a ticket, then get on the train and find seats. Buying tickets in advance does not affect the probability of you being able to sit together.
You should also not worry about connections, as on most routes trains run every half hour, and if you miss a train you take the next one. Taking a train from Interlaken to Zurich is as trivial as taking a train from Geelong to Melbourne. Do not overthink this.
If you are indeed coming up from Italy it may be good to buy the Swiss Half Fare Card in advance, and your tickets for the first trip. The SHFC can be bought at stations in Austria and Germany, but not AFAIK in Italy. The trip from Italy to your first base in Switzerland you can book on www.sbb.ch. And you actually do not need to already have your SHFC in order to buy those tickets. I will repeat this again: You do not need this card to buy tickets, you only need it to travel on them. During purchase you can just select “Half Fare Card” from the discount drop down box, and that is it.
Do not overthink this. This is mass transit. You have that in Australia as well. Would you plan train travel around Sydney months in advance?