Count how many days of travel you will have. Vienna to Tallinn can't be done in a single day, and neither can Tallinn to France, so you probably need at least 6 days.

A ‘7 days in 1 month Global Pass’ might work, as long as you will finish all your travel by 21 Jan.

Yes, you can buy it now and choose the start date later with an electronic railpass.

But

Trains are very slow and indirect in the Baltic States, and I think there's still no passenger trains at all between Lithuania and Latvia.

I traveled there a couple of months ago, and found that bus was the most effective way to travel between the capitals. Even if you do choose to take the train from Poland to Lithuania, and from Latvia to Estonia, tickets are cheap anyway.

And depending on where you will start in France, you may have to pay a heavy reservation fee to get to Switzerland.

So I would certainly consider a railpass for the part of your journey in Switzerland, Austria, Germany, Poland, Czechia, and maybe France, but I would probably but my own tickets in the Baltics.

See here for an excellent guide on getting to Tallinn.

It's written as a guide from London, but the part after Warsaw will be the same for you.

https://www.seat61.com/Estonia.htm#via%20Brussels

I've recently done an interrail through the Baltics. I just went from Tallinn first though 

I will echo @ralderton and say buses are better in the Baltics.

I'd advise getting to Warsaw or Krakow. From there you can get the train to Vilnius. It's a split train. The Polish train stops at a little station called Mockava. From there the Lithuanian train goes on to Vilnius.

From Vilnius you can get a Flix bus all the way to Tallinn, via Riga.

You can do it in one go. Current schedules would have you get in to Vilnius at 14.41. You'll have missed daytime Flix coaches but you could travel overnight and get in to Tallinn early the next day.

Would you consider flying from Vienna to Tallinn? Just did a quick search and Google gave me prices of £39.

There's a sleeper train from Vienna to Warsaw, but it doesn't arrive in time to take the once daily Warsaw-Mockava-Kaunas-Vilnius service.

I might be tempted to do Vienna-Warsaw-Białystock on Day 1 (using a pass), then join the train to Kaunas or Vilnius on Day 2 (buying a ticket).

You should arrive in time to make a bus connection to Riga, which is 4.5 hours. Tallinn would be another 4.5 hours the next day.

It'll be an easy journey when Rail Baltica is completed.

I think either way it's going to be a hell of a journey. If I was young I might be inclined to do a sleeper to Warsaw, join the train to Vilnius, Vilnius to Tallinn bus that night.

I think I worked out it could be done with time waiting at Warsaw overnight. This only goes to Mockava because, for some reason I still can’t find the Warsaw to Vilnius train when searching. That Mockava train does continue to Vilnius.

Depends how willing someone is to sleep in, first, Warsaw station and, second, the bus to Tallinn. If you’re going to be in Tallinn for 2 weeks it’d be worth it.

At my age I think I'd be flying it was cheap.