'Astronomy' is always a work in progress as I have found.
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Whats is the soil like around there, looks pretty dry and hard from the pix. Any Rock base ?
The fact that it has had water (heavy stuff) previously for long periods says it will probably be pretty compacted and stable beneath (mentioned earlier).
Go with the tripod to start with and work out what to do from there. I sat mine on concrete slabs for a while without a problem if you need a bit of height. You also have enough room to step well away from the setup while imaging unlike my shed which is very small. Wish I had your space (and horizon) to work with.
If you think it is not stable you could always punch a few holes through with a post hole borer and pour column foundations (upside down piers) to support a solid slab. An 8" slab on that i not going to move.