> Nope, there is another important thing that matters: some of the cancers tested are really hard to detect early by other means, and very lethal when discovered late.

You are arguing for testing everyone there. If you cannot detect them by other means, you need to test for them this way. And do it for everyone. You have already set up the unrealistic wargaming scenario. You picked pancreatic cancer as your example where you do have to test every 6 months at least, because if you do it more rarely, the disease progression is so fast that testing is useless. There are no specific risk groups for pancreatic cancer beyond a slight risk increase by "the usual all-cancer risk factors". Nothing to pick a test group by.

And a 99% overall false positive rate is easy to achieve, lot's of tests that are in use have this property if you just test everyone very frequently. Each instance of testing has an inherent risk of being a false positive, and if you repeat that for each person, their personal false-positive risk of course goes up with it. All tests that are used frequently have an asymptotic 100% false positive rate.