Pirita beach has a very small area for nudism, closer to the forest, and only on very hot days can you meet naked like-minded people. Usually there is no one there. There are about 300 meters of the official nudist sea beach in Pärnu. This is a historic "beach for ladies", so there are few men or families with children. In Tartu, where I live, there is a very small piece of the river beach - but even in hot weather there are no more than 10 nudists. However, this is enough to play pétanque sometimes))
As for saunas and pools, I recommend to go to Finland - it is really not customary to go into a sauna or a shower in swimming trunks there. Never. I saw how even 10-13 year old teenagers disciplinedly take their swimming trunks when they wash themselves in the shower. In a sauna it is unthinkable to meet someone in swimming trunks - it is simply impolite in Finland. Helsinki has a large and popular pool, where you can and should swim naked. But the schedule of visits is separate - for men and women.