There are rails all the way-they even also have trains on them and most likely the planner will not work as youre like so many waaaaaaaaaaaaay too early. For some global idea use a samedayofweek next week or so on bahn.de Forget that app as planner asap. Bahn.de is best for general EUR planning-but will also not show a thing if TrenItalia has not put anything in (remember that very 1st lesson on how computers work??)
It is indeed to Milano, then via/perhaps cambio in Genova along Liguria coast till border point Ventimiglia (20.000), then local SNCF=FR TER train, cannot even RES, ev 30/60 mins via Nice still along coast Mediterranee till Cannes or Grasse. makes also a stop in Monaco-but thats now in a tunnel without anyhting to see. Planners often also have specific trouble with these local trains.
You can in theory even do it without paying for RES=extra change in Verona and use InterRegionale, but even with IC the trip is not very speedy-can well take up to 10 hrs.
ALT-check timings yourself closer to travelday: direct Frecce till TORIno, then the quite scenic train via mountains to Tende-(train also for Ventimiglia), change there to only ¾ time daily TER to N.
In the past people looked at maps in timetables-these still have their merits. There are also bu quite unclear maps in the general info section of this site
Too early, Trenitalia is always later with it’s schedule. Sometimes it only opens for booking just a couple of weeks in advance. Trains in France for June are possibly also not available for consultation or reservation yet (Eurostar is usually open for booking). Italy has plenty of connections, and there’s always an alternative. So no need for panic, plenty of connections will be possible.
If you just check times between point A and B, for e.g. next week, on a similar day and time you want to travel, you get a good indication of what you can expect in June. Timetables won’t differ a lot, perhaps even not at all.
France, Italy, Spain,… their schedules cannot be consulted a long time ahead.
There is a 2 hourly Intercity service from Milan to Ventimiglia and as already posted then there is a 30 minute service along the Cote D 'azur to Cannes. Planners often try to send you via Turin, which means changes.
Venice to Milan also has a regular direct service, but trains operated by Italo are not included in the pass