Paris and then Czech Republic for 21 days? Or just Czech!

Twenty-one days just in Czech Republic would be too much for me, but some people are "slow travelers" who want to absorb each place more fully. What is your travel style? You could do a week in Paris and two weeks in Czech Republic, which is still plenty of time to venture out of Prague to other large towns like Brno and Olomouc plus to some of the smaller quaint towns like Telc and some of the towns in southern Bohemia. It should be easy and cheap to fly between Paris and Prague. Book an open jaw flight into Paris out of Prague or vice-versa.

Really up to you. There is no "stick to one country" rule. I've done trips with multiple countries on most of my European trips - has always worked out well for me.

Another option might be Poland and Czech Republic. Twenty-one days would be enough time for a still fairly relaxed tour of the highlights of both countries, but it wouldn't quite be a "slow travel" experience if that's what you seek.

I spent 5 weeks in Poland last year (still skipped a bunch of places) and I wouldn't recommend combining it with Czechia on a 3-week trip if you like in-depth visits to the places you go. I ended up having to cut Czechia really short because of the Schengen restriction. When I ran out of Schengen time, I flew from Prague to London, so you can certainly combine Czechia and Paris. I don't like flying mid-trip and will only do it when I have to, but if you value variety it sounds like a good plan. One extra flight isn't that big a deal.

For a 21 day trip, I would prefer to visit 3-4 cities. I like to travel to places that are somewhat in close proximity to each other for efficiency's sake. I'm talking about visiting Prague, Vienna and Budapest. Or, Berlin, Dresden, Prague and Vienna. Another good trip would be Munich, Salzburg, Vienna and either Budapest or Prague.
With the advent of budget European airlines, I also like to catch a city on the way home completely different than the other cities on the trip--like stopping in Copenhagen--or Lisbon--or Edinburgh.
Going from place to place by train is not too difficult as you can just see so much more. I'd be bored sticking to one place/country.

We just returned from a 3.5 week trip. We divided our time between Budapest, Vienna, Cesky Krumlov, Prague and Amsterdam. For us, the pace was perfect.

I think you'll be very pleasantly rewarded with 3 weeks combining Northern Czechia and Southern Poland, you could do a bit of a loop: starting at Prague, heading up to Wrocław, turning east to Kraków, and then travel south-west to Olomouc, then ending up back in Prague. Doing this route by car would allow one the greatest freedom to stop in the smaller towns and enjoy the natural scenery in the countryside along the way.

If you want to avoid "too much moving around," then the question is not how many countries but how many cities. With three weeks, Paris and Prague would be easy, and you could take day trips from both as you choose. So you'd change hotels just once on the trip. Plenty to see in both cities and plenty of nearby places to visit without having to move your stuff.

We spent 5 weeks in the Czech Republic and did NOT run out of things to do and see. It's a beautiful country with lots of different regions with so much to offer. We want to go back and see the parts we missed!

Thanks for the feedback
We are planning for August

Probably 7-10 days in France
And then Czech Republic for 14 days or more

This year we travelled in May for the month and we hit Paris to Colmar to Lugano to lake Como to Bologna and finished in Padua

We spent 6 nights in Paris and we really enjoyed the city. We stayed in the Marais which was perfect but I thought we’d try a different area maybe st Germaine Not sure .

We both felt tired at the end and thought 3 weeks would be a batter travel time to be away.

I also told myself that just plan 1 country. But I seem to feel if we spend longer times in each destination it might be ok to do 2 countries.

I was aware that they had a bad heat waive in Europe this year.

Thanks