I fear it's very complicated to determine this and I'm fine not automating this for now (and rely on human intelligence to determine if the version should bumped).
If we determine the breakingness however, I think we should be looking at the generated crd.yaml, not the Rust code. Maybe there are already some tools for that already ;)

I fear it's very complicated to determine this and I'm fine not automating this for now

It is indeed very complicated. I would even say impossible to do it reliably (right). I just left this to note down a random thought I had.

If we determine the breakingness however, I think we should be looking at the generated crd.yaml, not the Rust code. Maybe there are already some tools for that already ;)

Sure, that also makes sense. If we would down that road, I would still explore defining this in Rust code. This would also enforce specific rules around code. But as stated above, this was mostly just a random thought which will be removed from the final ADR anyway.