Just a short answer. It's bedtime here. It sounds like a nice honeymoon.

Many trains you can use without reservation. Night trains always require reservation.

In France there are high speed trains (TGV) which require reservation and regional trains, that take longer time, but without reservation.

In Switzerland no reservation is needed. There are some scenic trains which require reservation in the panorama cars but it is also possible to travel the same route without reservation. Pepole in Switzerland never make reservations as the trains run very frequently every day.

REServ are only needed for some trains=varies per country, mostly overnight and the hi-speed expresses. Use planner of bahn.com (german raiwlays_covers also most of EUR and will clearly show if REServ is needed, optional or not possible. Often its possible to avoid these by using slower regional trains (that will however often not have 1st cl-as it seems USAers are brainwashed to believe that is what to use).

Also DO be much aware as for now it is IMPossible to go by train via what once was Yugoslavija-the split-off countries have no more trains over their borders and what still runs is marginal and out of date.

Decide if you really want Zagreb (and then best to fly) or skip it and use ferry from IT to GR.

BTW-perhaps there are however old family ties or so: I would suggest to go via Wien=Vienna to Budapest, then maybe fly to GR (try WIZZair=a low cost airline=pay extra for anything beyond just a seat)