Hey,

if you are flexible there will always be some trains available… just it might take a little bit more time.
On your route actually only in Italy you will need compulsory reservations (except if you are going to use nighttrains) and maybe also to get to Split you will need a reservation. Beside you can just hop on the train and if all the seats are full, sit/stand on the floor…

About hostels: depends, this summer they might be full again in some places, especially on weekends and in popular cities like Venice etc. or if there is only one hostel available. But I wouldn’t worry too much about that - you might not be able to get a place in the most popular hostel in town but you still will find some accommodation for a low price usually (there might be exceptions as nobody can yet forecast how well this seasons will be going and also now there are again a lot of overseas traveller around, unlike the last two years).

It is not a point of now or never-its more  like ´what is best term to do advance´. With current sites/online etc you can always check what is available via sites like booking etc. The hoStels that belong to the Hi-hostel chain (like in Sverige the STF-dk about norsk) have their own site and are better if you have membership for that.

Populair touristy towns have dozens of hoStels-or likely HAD-as many were froced to close down (no people, no distance) due to covid. Plus that if you are in a couple-a cheap HTLroom often cost about same as 2 beds (but without the xtraś). Also look for longer stays to accomodations with cooking/airb&b etc.

And its indeed as MM above explains: you can do nearly all without REServing seats on trains-though it might be advisable on busy days to do it for a few €. MOst major routes have a train ev. hour=like naertrafik around Oslo. Just check what the timings are (roughly, indicative) on a planner like bahn,de-if it is not there yet for aug, then use samedayofweek in may or june. Nearly everywhere is (Much) cheaper as in NO.

For the trains in Switzerland there are no mandatory reservations and as the trains run very often you will most likely find a free seat. If you check the train you to plan to take on the SBB web site or in the SBB app there is a figure showing how full the train is expected to be. 

https://www.sbb.ch/en/home.html

A good place to stay in Bern is the Marthahaus:

https://www.marthahaus.ch/pages/hotel-en/20/?oid=1861&lang=en

When you stay in a hotel in Bern you will get free rides on the public transportation in Bern during your stay.