I think it would make a great bench top.
First, you didn’t say how cupped it is. Its possible to pull it back “maybe”. Early in the morning, on a sunny day, throw it on the wet grass cupped side down. I’ve never tried it with something this thick, but it may help.
The knots will make it harder to flatten, but also make it harder period, so you shouldn’t need to flatten that often. As for periodic flattening. How often to you think you’ll need to flatten it. Once every 25-30 years? That would be my guess for oak.
Resawing it and re-glueing would also work if you can’t get it flat. If its got less than an 1/2” cup though, I’d flatten it.
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