Potassium is everywhere, including in every living cell.

So the exposure to potassium radiation is constant for any living being on Earth.

Exposure to thorium and radon varies greatly depending on the location, i.e. it can be significant in places with granitic rocks or with sediments whose origin is in the erosion of granitic rocks.

So some people, animals etc. may be exposed to more thorium/radon radiation than from other sources, especially if inhaling dust or radon gas, but for most the greatest exposure is from the content of potassium 40 and carbon 14 that is inside their bodies (which produce only weak beta radiation).

Yes that is true, but most of potassium and carbon are not radio isotopes. All of thorium is radioactive. As such it is likely true that all life carries some quantity of radioactive potassium and radioactive carbon but that does not mean radioactive carbon or potassium are more abundant or available than thorium everywhere life exists.

In sea water the amount of thorium is negligible, because thorium minerals have very low solubility. So there radioactive potassium and carbon are much more abundant.

Averaged over the entire Earth, thorium is about one hundred thousand times less abundant than potassium, while potassium 40 is only ten thousand times less abundant than all potassium, so there is more potassium 40 than thorium. When the Earth condensed, there was much more K40 than today, so in the early Earth potassium produced more heat than thorium and uranium.

In the continental crust, more of the thorium of the entire Earth has been concentrated than the corresponding fraction of potassium, so thorium is only about 2300 times less than potassium by mass, so in the continental crust there is about 4 times more thorium than potassium 40 by mass.

However, rocks with potassium, like feldspars, are everywhere, while phosphates, which are the main minerals with abundant thorium, are more rarely encountered at the surface. So there are plenty of places where there is much more K40 than thorium or uranium.

Moreover, at equal mass potassium 40 is a few times more radioactive than either thorium or natural uranium, due to having both a higher probability to decay and to having more atoms per unit of mass. So even if potassium 40 is 4 times less abundant by mass than thorium in the continental crust, it generates several times more radioactive decay events per second. The energy of the radiation from K40 is lower, so overall thorium produces somewhat more heat in the crust.

While potassium generates more radiation than thorium, it is dispersed, there will never be in nature a place with concentrated K40, without being diluted with non-radioactive potassium. So any living being or any potassium mineral will never be as radioactive as a piece of rock that contains thorium minerals.

Moreover, if thorium remains enclosed in some mineral for thousands of years, that mineral accumulates the decay products of thorium, e.g. radium, polonium etc., becoming several times more radioactive than pure thorium. If such a thorium mineral is broken, the dust from it would be many times more radioactive than pure thorium.

So there is a risk that one may encounter a piece of something containing thorium with relatively high radioactivity, while there is no such risk with potassium 40, outside of some laboratory that would try to separate it from natural potassium (for which no good reason exists and such a purified K40 would be highly dangerous, because unlike with thorium or uranium, any living cell would attempt to incorporate such potassium).

In conclusion, while some animals, or more likely some humans, might encounter some thorium minerals and be irradiated by them, the radiation doses averaged over all living beings consist overwhelmingly of radiation from K40 and C14, augmented for marine living beings with a lower dose from the uranium dissolved in sea water. The radiation dose coming from thorium or from other weakly radioactive elements that are abundant, e.g. calcium and vanadium, is pretty negligible in comparison with those.