High speed /Intercity trains in France, and almost all in Spain require reservations. And in Spain, they can be awkward to make.

So my first tip would be to steer away from France and Spain, particularly if you want to stay flexible.

Switzerland and Austria are great - no reservations needed.

Trains in Croatia only go as far south as Split, but you can get a bus or ferry to Dubrovnik, and you can get ferries across to Italy.

Something like Paris - Strasbourg - Basel - Bern - Zurich - Innsbruck - Vienna - Ljubljana - Zagreb - Split - bus to Dubrovnik - ferry to Italy - Nice - Montpellier - Barcelona might be a good route, based on the countries you've mentioned. There’s a lot to see, so it’s just a suggestion.

Some advice :

I'd choose a 10 or 15 days within 2 months pass (whether 1st or 2nd class is up to you, the price difference is much less than with tickets).

Dubrovnik does not have a railway station. The nearest one connected to the European network is in Split. There are multiple buses between both cities.

There is a very convenient night train from Zagreb to Zurich. Depending on your budget there are seats (not recommended), couchettes in 4 or 6 people compartments and sleepers. The route is also lovely during the day but it'd take a full day.

The Eurail Pass is good value in Switzerland : most of the network is covered and regular tickets are expensive. Boats on Lake Thun and Brienz are included while mountain railways and cable cars aren't (Jungfraujoch, Titlis, Schilthorn). Interlaken Ost - Grindelwald/Lauterbrunnen isn't either. You do get a 25-50% discount on those though.

Seat reservations are mandatory on fast/high-speed trains in France and Spain. You'll have to book in advance in summer which reduces your flexibility.

Have a look at this link : https://www.seat61.com/interrail-and-eurail-reservations.htm

Connections between France and Spain are bad. Only 3-4 daily high-speed trains which sell out days if not weeks in advance. If you've got time you could take the slow route via the coast (reservation-free trains).

So the itinerary could look like :

- Dubrovnik (extremely crowded in summer but like most of the Mediterranean)

- Split

- Zagreb

- Zurich, Bern, etc.

- maybe Strasbourg

- Paris

- somewhere inbetween (Avignon, Nîmes, Montpellier) 

- Barcelona

- San Sebastian/Basque country (beautiful)

- Bordeaux idk

My only advice would be to spend at least 2 nights at each stop (except maybe Zagreb), don't rush it.

Thanks everyone! I really appreciate the helpful advice. Based on what’s been said above, it seems a good call to focus on Austria and Switzerland rather than France and Spain. 
 

Given I have roughly 17/18 days for the first leg, does the following sound like too much to squeeze in: Dubrovnik - Split - Zagreb - Ljubljana - Graz - Vienna - Zurich/Bern (with time to explore a couple of other Swiss towns on day trips) - Paris. Or does that sound doable?

It's maybe one or two cities too many for me, but it depends on how fast you like to go. You can easily lose most of a day if you're not careful, between checking out of your hotel, getting to the next city, finding the hotel, getting your bearings.

I might skip Zagreb if I had to cut one. I don't know much about Graz.

Basing yourself somewhere in Switzerland for a few days is a good idea. The rail network makes it excellent for day trips, and staying put for 3 or 4 nights is very welcome break compared to unpacking every day or two.

And the sleeper network is useful to know about to save time. (But don't reckon on getting a ton of sleep)

https://nachtzugkarte.de/en/

Thanks! I’d prefer not to be rushing from place to place so I’ll cut 1 or 2 out. Appreciate all the advice :)