Is flixbus a good travel option for Berlin to Prague travel next spring?

Depends on how you define good, personally there is no way I'd take a coach for such a long trip when there is a good train option.

In 2018 we took the Flix bus from Dresden to Prague. It was about a two hour ride and it was fine. In 2021 during a train strike we took the Flix bus from Berlin to Leipzig and it was fine too. We are also seniors in our 70s. The Dresden to Prague ride was much shorter than the train ride would have been. They do have a rest room on the bus.

I would never, ever do this. Long distance bus rides are uncomfortable. I prefer the high-speed train.

Try to see if Regiojet offers this route. RJ is 1000% better than Flixbus.

Regiojet indeed has buses between Berlin and Prague, although the travel time is almost exactly the same as Deutsche Bahn trains, maybe beats them by 5 minutes. DB price is not that much more, so I am wondering why the bus is preferred.

I recently used Flixbus between Vienna and Budapest. It was fine. The bus was on time, clean and comfortable. Didn't take much more time than the train, on that route.

Flixbus and Regiojet operate between Berlin and Prague, with pretty much same travel durations as the train. The busses are comfortable and usually not full. Bus option is a bit cheaper than train, but keep in mind they leave from the bus station located outside the city. You'd need to factor in the time and hassle of using public transport/taxi to get there.

I had to book a last-minute Flixbus ticket from Dresden to Prague last week due to problems with the trains between the two cities. Very comfortable, fast trip on the motorway, but not as scenic as the train. Also the bus hangs around a bit at the bus station on the edge of the centre at Florenc, before going on to Hlavni station closer to the centre. Bit of a disturbance coming into Prague as an old guy on the bus had been smoking in the toilet and set off the detector, the co-driver was threatening to throw him off the bus and he was ranting about going to the European Court of Human Rights.

We travelled on FlixBus in France recently (route was complex by train and the bus was direct so it seemed like a good idea). The pickup point was on a busy road 3 km from the the city centre, no shelter from the rain or toilets nearby, the bus was 1.5 hours late - only notified incrementally as we were waiting. On board was a notice that the toilet was not to be used. On the plus side, it was cheap and we reached our destination eventually. Trains are so much better if that choice is available.