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Chris has demonstrated the reason for piers in clay. His slab moves around as it is not founded on deep piers but in the top layer. Not a good base for any building you would like to stay level.
Irrespective of the size and weight of the pier, one pier by itself will heave and move around with the soil over time. You can only found it deeper or add a number of piers anchored into rock, but as you say , it is not a bridge.
The idea of drainage well away from the pier may help isolate the top layer of wet soil outside from the inside dry material as long as it is sealed and does not work in reverse and let water deeper into the top layer. 500 sounds pretty deep. For that amount of digging you may as well replace the soil under your observatory with a 200 bed of hardcore which is unaffected by water or fill the slot with concrete and put your observatory on it.
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