My understanding is non rad hardened method get around this by basically doubling or some multiple of repeating calculations and chexking data often.

Random errors will occur you just need to be checking fast enough to fix and update that bad bit flip.

I am sure there's all sorts of fun algorithms in this space but I am under the impression there is SOME tax to doing this. What is the tax? Is it 10% ir 60% I have no idea would love to know!

Why make a GW datacenter on the ground if you can make two and pay to launch them into space?

There's more than that, it's possible to get permanent hardware damage from radiation at smaller (modern standard) process sizes.