Unless you have access to the inside of the walls and can go in and replace all the romex to 12awg youre only option is to go out and buy 15amp breakers.

The reason the previous homeowners bought 20 amp was because they kept tripping the breakers and thought 20amp would work which it did but they are super lucky the 14awg didnt overheat and catch fire.

Although its worth knowing that 14awg is actually manufactured to withstand higher amps that what NEC requires. But it makes sense because NEC does not want to push it to the limit for every rule. Thats just asking for problems. They usually suggest loading a circuit only to 85%. But I wouldnt do that if I were you just because the wire technically can handle it.

If I were you id go buy 15 amp breakers for peace of mind and to get it back to proper code.

30 amp is normal for dryer circuits. Should be at least #10.