I can really recommend the spa Therme Erding near Munich in Germany.
https://www.therme-erding.deI was there for the 3rd time but I just saw that there was no review about it here yet.
According to their website it's the world's largest spa. Unfortunately not entirely naturist but as almost everywhere in Germany, the sauna area is naked area. And it is not only a few saunas but 27 different saunas and many warm water pools with lots of activities and actions. There are many themes, e.g. bakery sauna where you they bake bread rolls and you get them after the 15 minutes sweating, there is another where you get Erdinger Weissbier (alcohol free), there are some where you get salt or other kind of peeling/cream for your skin, and of course show ceremonies with music and entertainment (I very like the Russian sauna with the Russian style music and very wet show).
And many warm water pools, mostly I think with 34 °C, with whirlpools, massaging water, pool bar... And many sunbeds and sleeping rooms to have rest. Also many restaurants where you pay with your armband key (chip). It's also the locker key and you pay at the exit.
People don't run around naked, it's mostly limited to the time before/after saunas and pools, most people wear a towel around or bathrobes if they move around or sit at the restaurants or bars (except the pool bar because you sit naked in the water:) )
There is a long timetable of the events, there is something to do almost every 15-30 minutes and in the peak afternoon hours there are even 4-6 events in different saunas at the same time. However even then it can be crowded, for the popular shows you need to arrive about 10 minutes before the start. But it's worth!
It's only for adults over 16 years, so no kids are allowed in this area but many young adults are there.
For all age groups there are the other parts of the spa, that are with swimsuits. E.g. there is a big wave pool, many other pools and lot of fast water slides.
There are so many things to do that it is better to go for more days, we usually stay for 3 days and it's also not enough:)
If you have questions about this spa, I could answer, I'm almost an "expert" visitor:)