The FKK girls are not tested. I know that some get checkups regularly on their own initiative.
But it is a fairly clean, safe environment for paid sex. Otherwise I can assure you I wouldn't be there. There are hundreds (thousands?) of guys who have been doing everything (except bareback penetration) for years and never got anything beyond a common cold. I myself once got a throat infection, probably from an FKK girl, but which had nothing to do with sex.
How can I possibly give you percentage chances anyway? I don't know. Would it change your mind if I said 4.26%? Or would you feel more comfortable if I said 0.426%? There's still no guarantee.
If you are worried about things like DFK, DATY and BBBJ, no one is forcing you to partake. Continue to insist on no kissing, no DATY and CBJ. The girls have no problem with your wishes, they just want a disease-free EUR 50 note, please.
If you are so worried about these things, it is probably better to stay in your home town, (where you probably have a higher statistical chance of being run over by the school bus).